feat: faster ui (closes #171, closes #128, closes #89)

* Replaced the NSCollectionView with a custom NSView. Benefit includes simpler code, and control over recycling logic.
* In cases where the user has many open windows, replaced the downscaling strategy with a scrollable view
* Added a preference: align thumbnails to the left or center

alt-tab-macos.xcodeproj/project.pbxproj

@@ -12,11 +12,11 @@
 		D04BA004884A273D4D2D3EF1 /* HelperExtensions.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAD91161791D42FEC4A60 /* HelperExtensions.swift */; };
 		D04BA084CD1236EC78D90A01 /* Localizable.strings in Resources */ = {isa = PBXBuildFile; fileRef = D04BACCBE5F97BE9B6CA645B /* Localizable.strings */; };
 		D04BA100BD0F47828EB649FF /* InfoPlist.strings in Resources */ = {isa = PBXBuildFile; fileRef = D04BAAEC2847830A3991F8D1 /* InfoPlist.strings */; };
+		D04BA11E56383D082D7BE5A5 /* ThumbnailsView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAA998119CAA8B70A2B67 /* ThumbnailsView.swift */; };
 		D04BA14D93726795A6937832 /* LabelAndControl.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA2526DC6726E0F7ACF7C /* LabelAndControl.swift */; };
 		D04BA15A1B0C4871EA7CB899 /* GeneralTab.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BACE22DC907F03D193075 /* GeneralTab.swift */; };
-		D04BA1B133D53572D7B312C2 /* CollectionViewItemFontIcon.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA1DF8CAB2FAB7FE9244B /* CollectionViewItemFontIcon.swift */; };
-		D04BA1CEC6B9C8945FEC8740 /* CollectionViewItemView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA258B56193958D60978A /* CollectionViewItemView.swift */; };
-		D04BA263682C4EA2F85B6CF0 /* QA.md in Sources */ = {isa = PBXBuildFile; fileRef = D04BACAE340252C1E910D3CE /* QA.md */; };
+		D04BA1B133D53572D7B312C2 /* ThumbnailFontIconView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA1DF8CAB2FAB7FE9244B /* ThumbnailFontIconView.swift */; };
+		D04BA1CEC6B9C8945FEC8740 /* ThumbnailView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA258B56193958D60978A /* ThumbnailView.swift */; };
 		D04BA26A691D56031FCCF00C /* Sysctl.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA8DB8AA7E5570DAC568A /* Sysctl.swift */; };
 		D04BA276B3241D440F65B149 /* InfoPlist.strings in Resources */ = {isa = PBXBuildFile; fileRef = D04BA5C2BB394F1624DD5B45 /* InfoPlist.strings */; };
 		D04BA2A6FF9DDDC5A1A68E36 /* Applications.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA282BB16C1554595A968 /* Applications.swift */; };
@@ -30,12 +30,10 @@
 		D04BA5F99B45DC13B9E9DD91 /* Keyboard.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA8276B3D3905E80B1739 /* Keyboard.swift */; };
 		D04BA6187A91A847844B6ABB /* Window.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA015A45DE7AFDC9794FE /* Window.swift */; };
 		D04BA691CB6082A3C39CBC89 /* TabViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAE757BB2B605234FBF58 /* TabViewController.swift */; };
-		D04BA69D47B5E60A6AD9CBD9 /* CollectionViewItemTitle.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAD1297730B191E96E7FE /* CollectionViewItemTitle.swift */; };
-		D04BA6C953494839648107D1 /* CollectionViewItem.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA2A4A4140AF3E09DA94D /* CollectionViewItem.swift */; };
+		D04BA69D47B5E60A6AD9CBD9 /* ThumbnailTitleView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAD1297730B191E96E7FE /* ThumbnailTitleView.swift */; };
 		D04BA737008AA2CD4E230A21 /* Application.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA10777505D8A67ABD186 /* Application.swift */; };
 		D04BA73E90EFEF8247A5105D /* CGWindow.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAC34CFD42A7F6F1F01C0 /* CGWindow.swift */; };
 		D04BA76A74267B1346D23687 /* GridView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA6D57A1456C07318B8EA /* GridView.swift */; };
-		D04BA76DDB00FC50D203D62C /* CollectionViewFlowLayout.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BAC2FEF7248B7BF9579E2 /* CollectionViewFlowLayout.swift */; };
 		D04BA775CF3F8D9394A1E256 /* Screen.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA68C2561D9EE4FD851B8 /* Screen.swift */; };
 		D04BA7BE7F3DD24D58ACE942 /* AppearanceTab.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA64F1F344007EA13BA05 /* AppearanceTab.swift */; };
 		D04BA7F86F1926FBE31F44BF /* BaseLabel.swift in Sources */ = {isa = PBXBuildFile; fileRef = D04BA53992F116E5E704CAB3 /* BaseLabel.swift */; };
@@ -85,15 +83,14 @@
 		D04BA123744B0C27E9F54B05 /* codesign_sparkle_embedded_apps.sh */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.script.sh; path = codesign_sparkle_embedded_apps.sh; sourceTree = "<group>"; };
 		D04BA1C3E42AC44CA2C5D3D8 /* app-icon.svg */ = {isa = PBXFileReference; lastKnownFileType = file.svg; path = "app-icon.svg"; sourceTree = "<group>"; };
 		D04BA1D80F4EEF2A91BAD29C /* release.config.js */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.javascript; path = release.config.js; sourceTree = "<group>"; };
-		D04BA1DF8CAB2FAB7FE9244B /* CollectionViewItemFontIcon.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CollectionViewItemFontIcon.swift; sourceTree = "<group>"; };
+		D04BA1DF8CAB2FAB7FE9244B /* ThumbnailFontIconView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ThumbnailFontIconView.swift; sourceTree = "<group>"; };
 		D04BA1FC9022590D7AA02486 /* 1 window - 1 line.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "1 window - 1 line.jpg"; sourceTree = "<group>"; };
 		D04BA2526DC6726E0F7ACF7C /* LabelAndControl.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = LabelAndControl.swift; sourceTree = "<group>"; };
-		D04BA258B56193958D60978A /* CollectionViewItemView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CollectionViewItemView.swift; sourceTree = "<group>"; };
+		D04BA258B56193958D60978A /* ThumbnailView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ThumbnailView.swift; sourceTree = "<group>"; };
 		D04BA26154AB2A2897E08CAF /* windows-theme.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "windows-theme.jpg"; sourceTree = "<group>"; };
 		D04BA26C75F76C277653C932 /* FeedbackWindow.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FeedbackWindow.swift; sourceTree = "<group>"; };
 		D04BA27C87B86C4484A5B15B /* TabViewItem.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TabViewItem.swift; sourceTree = "<group>"; };
 		D04BA282BB16C1554595A968 /* Applications.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Applications.swift; sourceTree = "<group>"; };
-		D04BA2A4A4140AF3E09DA94D /* CollectionViewItem.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CollectionViewItem.swift; sourceTree = "<group>"; };
 		D04BA2A4F257F4DCE1421758 /* Podfile.lock */ = {isa = PBXFileReference; lastKnownFileType = file.lock; path = Podfile.lock; sourceTree = "<group>"; };
 		D04BA2C7B51F68651B3C60E2 /* 6 windows - 1 line.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "6 windows - 1 line.jpg"; sourceTree = "<group>"; };
 		D04BA32F25860B686DFE818A /* 3 windows - 1 line.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "3 windows - 1 line.jpg"; sourceTree = "<group>"; };
@@ -133,6 +130,7 @@
 		D04BA9B93823398A542FF7A0 /* Preferences.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Preferences.swift; sourceTree = "<group>"; };
 		D04BA9EF65B2E7AF9E3ADCA3 /* 2 windows - 1 line.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "2 windows - 1 line.jpg"; sourceTree = "<group>"; };
 		D04BAA34E0CB00DED7C04B4F /* 2-rows.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "2-rows.jpg"; sourceTree = "<group>"; };
+		D04BAA998119CAA8B70A2B67 /* ThumbnailsView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ThumbnailsView.swift; sourceTree = "<group>"; };
 		D04BAA9E0539EE620D08F63F /* fr */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.strings; name = fr; path = InfoPlist.strings; sourceTree = "<group>"; };
 		D04BAAB92261FC04854FDDE9 /* App.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = App.swift; sourceTree = "<group>"; };
 		D04BAAF760E3A8A22BDA84D6 /* appcast.xml */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xml; path = appcast.xml; sourceTree = "<group>"; };
@@ -142,7 +140,6 @@
 		D04BABFEC8F9DF41BB7A449E /* import_codesign_certificate_into_keychain.sh */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.script.sh; path = import_codesign_certificate_into_keychain.sh; sourceTree = "<group>"; };
 		D04BAC02D60EF22D9CC7D969 /* commitlint.config.js */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.javascript; path = commitlint.config.js; sourceTree = "<group>"; };
 		D04BAC159731F80FDAF4EA6C /* 1-row.jpg */ = {isa = PBXFileReference; lastKnownFileType = image.jpeg; path = "1-row.jpg"; sourceTree = "<group>"; };
-		D04BAC2FEF7248B7BF9579E2 /* CollectionViewFlowLayout.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CollectionViewFlowLayout.swift; sourceTree = "<group>"; };
 		D04BAC2FF99F629CD4ED20FC /* MainMenu.xib */ = {isa = PBXFileReference; lastKnownFileType = file.xib; path = MainMenu.xib; sourceTree = "<group>"; };
 		D04BAC34CFD42A7F6F1F01C0 /* CGWindow.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CGWindow.swift; sourceTree = "<group>"; };
 		D04BAC6AFC7F06D1A567F27A /* replace_environment_variables_in_app.sh */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.script.sh; path = replace_environment_variables_in_app.sh; sourceTree = "<group>"; };
@@ -152,7 +149,7 @@
 		D04BACD976030676FD0761D5 /* Windows.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Windows.swift; sourceTree = "<group>"; };
 		D04BACE22DC907F03D193075 /* GeneralTab.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = GeneralTab.swift; sourceTree = "<group>"; };
 		D04BACEE8D430B8CAAD8C4CD /* BoldLabel.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BoldLabel.swift; sourceTree = "<group>"; };
-		D04BAD1297730B191E96E7FE /* CollectionViewItemTitle.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CollectionViewItemTitle.swift; sourceTree = "<group>"; };
+		D04BAD1297730B191E96E7FE /* ThumbnailTitleView.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ThumbnailTitleView.swift; sourceTree = "<group>"; };
 		D04BAD241A6928F45355B315 /* es */ = {isa = PBXFileReference; fileEncoding = 2483028224; lastKnownFileType = text.plist.strings; name = es; path = Localizable.strings; sourceTree = "<group>"; };
 		D04BAD40CE2D3A8AAC3819D0 /* .gitignore */ = {isa = PBXFileReference; lastKnownFileType = file.gitignore; path = .gitignore; sourceTree = "<group>"; };
 		D04BAD60C97E609A759E721E /* UpdatesTab.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = UpdatesTab.swift; sourceTree = "<group>"; };
@@ -375,12 +372,11 @@
 		D04BA62E30174C336E4080FA /* main-window */ = {
 			isa = PBXGroup;
 			children = (
-				D04BAC2FEF7248B7BF9579E2 /* CollectionViewFlowLayout.swift */,
-				D04BA2A4A4140AF3E09DA94D /* CollectionViewItem.swift */,
 				D04BA653BD073CB58E2CFC93 /* ThumbnailsPanel.swift */,
-				D04BAD1297730B191E96E7FE /* CollectionViewItemTitle.swift */,
-				D04BA1DF8CAB2FAB7FE9244B /* CollectionViewItemFontIcon.swift */,
-				D04BA258B56193958D60978A /* CollectionViewItemView.swift */,
+				D04BAD1297730B191E96E7FE /* ThumbnailTitleView.swift */,
+				D04BA1DF8CAB2FAB7FE9244B /* ThumbnailFontIconView.swift */,
+				D04BA258B56193958D60978A /* ThumbnailView.swift */,
+				D04BAA998119CAA8B70A2B67 /* ThumbnailsView.swift */,
 			);
 			path = "main-window";
 			sourceTree = "<group>";
@@ -678,12 +674,10 @@
 				D04BA3CF766857381519B892 /* DispatchQueues.swift in Sources */,
 				D04BA48B00B4211A465C7337 /* DebugProfile.swift in Sources */,
 				D04BAB68B7B8D1B548BC3AD5 /* App.swift in Sources */,
-				D04BA76DDB00FC50D203D62C /* CollectionViewFlowLayout.swift in Sources */,
-				D04BA6C953494839648107D1 /* CollectionViewItem.swift in Sources */,
 				D04BAB048DE698E013577C51 /* ThumbnailsPanel.swift in Sources */,
-				D04BA69D47B5E60A6AD9CBD9 /* CollectionViewItemTitle.swift in Sources */,
-				D04BA1B133D53572D7B312C2 /* CollectionViewItemFontIcon.swift in Sources */,
-				D04BA1CEC6B9C8945FEC8740 /* CollectionViewItemView.swift in Sources */,
+				D04BA69D47B5E60A6AD9CBD9 /* ThumbnailTitleView.swift in Sources */,
+				D04BA1B133D53572D7B312C2 /* ThumbnailFontIconView.swift in Sources */,
+				D04BA1CEC6B9C8945FEC8740 /* ThumbnailView.swift in Sources */,
 				D04BA3BFB0CDF4ED343914B2 /* PreferencesWindow.swift in Sources */,
 				D04BA691CB6082A3C39CBC89 /* TabViewController.swift in Sources */,
 				D04BA14D93726795A6937832 /* LabelAndControl.swift in Sources */,
@@ -700,7 +694,7 @@
 				D04BAF25E67A5B31CF7676DB /* TextField.swift in Sources */,
 				D04BAD2A7F2E8BF64EE982E9 /* TextArea.swift in Sources */,
 				D04BA7F86F1926FBE31F44BF /* BaseLabel.swift in Sources */,
-				D04BA263682C4EA2F85B6CF0 /* QA.md in Sources */,
+				D04BA11E56383D082D7BE5A5 /* ThumbnailsView.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

config/base.xcconfig

@@ -6,7 +6,6 @@ MACOSX_DEPLOYMENT_TARGET = 10.12
 SWIFT_VERSION = 4.2
 INFOPLIST_FILE = Info.plist
 CODE_SIGN_ENTITLEMENTS = alt_tab_macos.entitlements
-IDEDerivedDataPathOverride = DerivedData
 FRAMEWORK_SEARCH_PATHS[config=*] = $(inherited) /System/Library/PrivateFrameworks // for SkyLight.framework
 ENABLE_HARDENED_RUNTIME = YES // for notarization
 OTHER_CODE_SIGN_FLAGS = --timestamp --deep --options runtime // for notarization

src/api-wrappers/HelperExtensions.swift

@@ -74,15 +74,23 @@ extension NSObject {
 
 extension Array where Element == Window {
     func firstIndexThatMatches(_ element: AXUIElement) -> Self.Index? {
-        // `CFEqual` is safer than comparing `CGWindowID` because it will succeed even if the window is deallocated
+        // == is safer than comparing `CGWindowID` because it will succeed even if the window is deallocated
         // by the OS, in which case the `CGWindowID` will be `-1`
-        return firstIndex(where: { CFEqual($0.axUiElement, element) })
+        return firstIndex(where: { $0.axUiElement == element })
     }
 
-    func firstWindowThatMatches(_ element: AXUIElement) -> Window? {
+    func firstWindowThatMatches(_ element: AXUIElement) -> Element? {
         guard let index = firstIndexThatMatches(element) else { return nil }
         return self[index]
     }
+
+    mutating func insertAndScaleRecycledPool(_ elements: [Element], at i: Int) {
+        insert(contentsOf: elements, at: i)
+        let neededRecycledViews = count - ThumbnailsView.recycledViews.count
+        if neededRecycledViews > 0 {
+            (1...neededRecycledViews).forEach { _ in ThumbnailsView.recycledViews.append(ThumbnailView()) }
+        }
+    }
 }
 
 extension NSView {
@@ -164,3 +172,10 @@ extension NSImage {
         return img
     }
 }
+
+// only assign if different; useful for performance
+func assignIfDifferent<T: Equatable>(_ a: UnsafeMutablePointer<T>, _ b: T) {
+    if a.pointee != b {
+        a.pointee = b
+    }
+}

src/logic/Application.swift

@@ -48,9 +48,9 @@ class Application: NSObject {
     }
 
     func observeNewWindows() {
-        let newWindows = axUiElement!.windows()?
-                .filter { $0.isActualWindow(runningApplication) && Windows.list.firstIndexThatMatches($0) == nil } ?? []
-        addWindows(newWindows)
+        if let windows = axUiElement!.windows(), windows.count > 0 {
+            addWindows(windows.filter { $0.isActualWindow(runningApplication) && Windows.list.firstIndexThatMatches($0) == nil })
+        }
     }
 
     override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
@@ -59,8 +59,11 @@ class Application: NSObject {
         addAndObserveWindows()
     }
 
-    private func addWindows(_ windows: [AXUIElement]) {
-        Windows.list.insert(contentsOf: windows.map { Window($0, self) }, at: 0)
+    private func addWindows(_ axWindows: [AXUIElement]) {
+        let windows = axWindows.map { Window($0, self) }
+        Windows.list.insertAndScaleRecycledPool(windows, at: 0)
+        windows.forEach { _ in Windows.moveFocusedWindowIndexAfterWindowCreatedInBackground() }
+        (App.shared as! App).refreshOpenUi()
     }
 
     private func observeEvents() {
@@ -101,11 +104,12 @@ private func eventApplicationActivated(_ app: App, _ element: AXUIElement) {
           let appFocusedWindow = element.focusedWindow(),
           let existingIndex = Windows.list.firstIndexThatMatches(appFocusedWindow) else { return }
     Windows.list.insert(Windows.list.remove(at: existingIndex), at: 0)
+    app.refreshOpenUi()
 }
 
 private func eventApplicationHiddenOrShown(_ app: App, _ element: AXUIElement, _ type: String) {
     for window in Windows.list {
-        guard CFEqual(window.application.axUiElement!, element) else { continue }
+        guard window.application.axUiElement! == element else { continue }
         window.isHidden = type == kAXApplicationHiddenNotification
     }
     app.refreshOpenUi()
@@ -116,10 +120,8 @@ private func eventWindowCreated(_ app: App, _ element: AXUIElement, _ applicatio
     // a window being un-minimized can trigger kAXWindowCreatedNotification
     guard Windows.list.firstIndexThatMatches(element) == nil else { return }
     let window = Window(element, application)
-    Windows.list.insert(window, at: 0)
+    Windows.list.insertAndScaleRecycledPool([window], at: 0)
     Windows.moveFocusedWindowIndexAfterWindowCreatedInBackground()
-    // TODO: find a better way to get thumbnail of the new window
-    window.refreshThumbnail()
     app.refreshOpenUi()
 }
 
@@ -127,4 +129,5 @@ private func eventFocusedWindowChanged(_ app: App, _ element: AXUIElement) {
     guard !app.appIsBeingUsed,
           let existingIndex = Windows.list.firstIndexThatMatches(element) else { return }
     Windows.list.insert(Windows.list.remove(at: existingIndex), at: 0)
+    app.refreshOpenUi()
 }

src/logic/Applications.swift

@@ -56,11 +56,17 @@ class Applications {
     static func removeRunningApplications(_ runningApps: [NSRunningApplication]) {
         for runningApp in runningApps {
             Applications.list.removeAll(where: { $0.runningApplication.isEqual(runningApp) })
+            var indexesToRemove = [Int]()
+            Windows.list.enumerated().forEach { (index, window) in
+                if window.application.runningApplication.isEqual(runningApp) {
+                    indexesToRemove.append(index)
+                }
+            }
             Windows.list.removeAll(where: { $0.application.runningApplication.isEqual(runningApp) })
         }
         guard Windows.list.count > 0 else { (App.shared as! App).hideUi(); return }
         // TODO: implement of more sophisticated way to decide which thumbnail gets focused on app quit
-        Windows.focusedWindowIndex = 1
+        Windows.updateFocusedWindowIndex(1)
         (App.shared as! App).refreshOpenUi()
     }
 

src/logic/Keyboard.swift

@@ -27,14 +27,20 @@ func listenToGlobalKeyboardEvents(_ app: App) {
     }
 }
 
-func dispatchWork(_ application: App, _ uiWorkShouldBeDone: Bool, _ fn: @escaping () -> Void) -> Unmanaged<CGEvent>? {
-    application.uiWorkShouldBeDone = uiWorkShouldBeDone
+func dispatchWork(_ fn: @escaping () -> Void) -> Unmanaged<CGEvent>? {
+    (App.shared as! App).uiWorkShouldBeDone = true
     DispatchQueue.main.async {
         fn()
     }
     return nil // previously focused app should not receive keys
 }
 
+func dispatchWorkCurrentThread(_ fn: @escaping () -> Void) -> Unmanaged<CGEvent>? {
+    (App.shared as! App).uiWorkShouldBeDone = false
+    fn()
+    return nil // previously focused app should not receive keys
+}
+
 func keyboardHandler(proxy: CGEventTapProxy, type: CGEventType, event_: CGEvent, appPointer: UnsafeMutableRawPointer?) -> Unmanaged<CGEvent>? {
     let app = Unmanaged<App>.fromOpaque(appPointer!).takeUnretainedValue()
     if type == .keyDown || type == .keyUp || type == .flagsChanged {
@@ -46,19 +52,19 @@ func keyboardHandler(proxy: CGEventTapProxy, type: CGEventType, event_: CGEvent,
             let isLeftArrow = event.keyCode == kVK_LeftArrow
             let isEscape = event.keyCode == kVK_Escape
             if type == .keyDown && isEscape && app.appIsBeingUsed {
-                return dispatchWork(app, false, { app.hideUi() })
+                return dispatchWorkCurrentThread { app.hideUi() }
             } else if isMetaDown && type == .keyDown {
                 if isTab && event.modifierFlags.contains(.shift) {
-                    return dispatchWork(app, true, { app.showUiOrCycleSelection(-1) })
+                    return dispatchWork { app.showUiOrCycleSelection(-1) }
                 } else if isTab {
-                    return dispatchWork(app, true, { app.showUiOrCycleSelection(1) })
+                    return dispatchWork { app.showUiOrCycleSelection(1) }
                 } else if isRightArrow && app.appIsBeingUsed {
-                    return dispatchWork(app, true, { app.cycleSelection(1) })
+                    return dispatchWork { app.cycleSelection(1) }
                 } else if isLeftArrow && app.appIsBeingUsed {
-                    return dispatchWork(app, true, { app.cycleSelection(-1) })
+                    return dispatchWork { app.cycleSelection(-1) }
                 }
             } else if isMetaChanged && !isMetaDown {
-                return dispatchWork(app, false, { app.focusTarget() })
+                return dispatchWorkCurrentThread { app.focusTarget() }
             }
         }
     } else if type == .tapDisabledByUserInput || type == .tapDisabledByTimeout {

src/logic/Preferences.swift

@@ -5,11 +5,11 @@ let defaults = UserDefaults.standard
 
 class Preferences {
     // default values
-    static var defaultValues: [String : Any] = [
+    static var defaultValues: [String: Any] = [
         "maxScreenUsage": Float(80),
         "minCellsPerRow": Float(5),
         "maxCellsPerRow": Float(10),
-        "minRows": Float(3),
+        "rowsCount": Float(3),
         "iconSize": Float(32),
         "fontHeight": Float(15),
         "tabKeyCode": kVK_Tab,
@@ -17,6 +17,7 @@ class Preferences {
         "metaKey": MacroPreferences.metaKeyList.keys.first!,
         "theme": MacroPreferences.themeList.keys.first!,
         "showOnScreen": MacroPreferences.showOnScreenList.keys.first!,
+        "alignThumbnails": MacroPreferences.alignThumbnailsList.keys.first!,
         "hideSpaceNumberLabels": false,
         "startAtLogin": true,
     ]
@@ -34,7 +35,7 @@ class Preferences {
     static var maxScreenUsage: CGFloat { CGFloat(defaults.float(forKey: "maxScreenUsage") / 100) }
     static var minCellsPerRow: CGFloat { CGFloat(defaults.float(forKey: "minCellsPerRow")) }
     static var maxCellsPerRow: CGFloat { CGFloat(defaults.float(forKey: "maxCellsPerRow")) }
-    static var minRows: CGFloat { CGFloat(defaults.float(forKey: "minRows")) }
+    static var rowsCount: CGFloat { CGFloat(defaults.float(forKey: "rowsCount")) }
     static var iconSize: CGFloat { CGFloat(defaults.float(forKey: "iconSize")) }
     static var fontHeight: CGFloat { CGFloat(defaults.float(forKey: "fontHeight")) }
     static var tabKeyCode: UInt16 { UInt16(defaults.integer(forKey: "tabKeyCode")) }
@@ -46,6 +47,7 @@ class Preferences {
     static var theme: Theme { MacroPreferences.themeList[defaults.string(forKey: "theme")!]! }
     static var metaKey: MetaKey { MacroPreferences.metaKeyList[defaults.string(forKey: "metaKey")!]! }
     static var showOnScreen: ShowOnScreenPreference { MacroPreferences.showOnScreenList[defaults.string(forKey: "showOnScreen")!]! }
+    static var alignThumbnails: AlignThumbnailsPreference { MacroPreferences.alignThumbnailsList[defaults.string(forKey: "alignThumbnails")!]! }
 
     // derived values
     static var cellBorderWidth: CGFloat { theme.cellBorderWidth }
@@ -100,6 +102,11 @@ enum ShowOnScreenPreference {
     case mouse
 }
 
+enum AlignThumbnailsPreference {
+    case left
+    case center
+}
+
 // macros are collection of values derived from a single key
 // we don't want to store every value in UserDefaults as the user could change them and contradict the macro
 class MacroPreferences {
@@ -116,4 +123,8 @@ class MacroPreferences {
         "Main screen": ShowOnScreenPreference.main,
         "Screen including mouse": ShowOnScreenPreference.mouse,
     ]
+    static let alignThumbnailsList = [
+        "Center": AlignThumbnailsPreference.center,
+        "Left": AlignThumbnailsPreference.left,
+    ]
 }

src/logic/Window.swift

@@ -1,6 +1,7 @@
 import Cocoa
 
 class Window {
+//    weak var itemView: CollectionViewItemView?
     var cgWindowId: CGWindowID
     var title: String
     var thumbnail: NSImage?
@@ -19,6 +20,7 @@ class Window {
         kAXTitleChangedNotification,
         kAXWindowMiniaturizedNotification,
         kAXWindowDeminiaturizedNotification,
+        kAXWindowResizedNotification,
     ]
 
     static func stopSubscriptionRetries(_ notification: String, _ cgWindowId: CGWindowID) {
@@ -57,8 +59,7 @@ class Window {
     }
 
     func refreshThumbnail() {
-        guard (App.shared as! App).appIsBeingUsed,
-              let cgImage = cgWindowId.screenshot() else { return }
+        guard let cgImage = cgWindowId.screenshot() else { return }
         thumbnail = NSImage(cgImage: cgImage, size: NSSize(width: cgImage.width, height: cgImage.height))
     }
 
@@ -121,6 +122,7 @@ private func axObserverCallback(observer: AXObserver, element: AXUIElement, noti
         case kAXUIElementDestroyedNotification: eventWindowDestroyed(app, element)
         case kAXWindowMiniaturizedNotification, kAXWindowDeminiaturizedNotification: eventWindowMiniaturizedOrDeminiaturized(app, element, type)
         case kAXTitleChangedNotification: eventWindowTitleChanged(app, element)
+        case kAXWindowResizedNotification: eventWindowResized(app, element)
         default: return
     }
 }
@@ -134,20 +136,22 @@ private func eventWindowDestroyed(_ app: App, _ element: AXUIElement) {
 }
 
 private func eventWindowMiniaturizedOrDeminiaturized(_ app: App, _ element: AXUIElement, _ type: String) {
-    guard let window = Windows.list.firstWindowThatMatches(element) else { return }
+    guard let index = Windows.list.firstIndexThatMatches(element) else { return }
+    let window = Windows.list[index]
     window.isMinimized = type == kAXWindowMiniaturizedNotification
-    // TODO: find a better way to get thumbnail of the new window (when AltTab is triggered min/demin animation)
-    window.refreshThumbnail()
     app.refreshOpenUi()
 }
 
 private func eventWindowTitleChanged(_ app: App, _ element: AXUIElement) {
-    guard let window = Windows.list.firstWindowThatMatches(element),
-          let newTitle = window.axUiElement.title(),
+    guard let index = Windows.list.firstIndexThatMatches(element) else { return }
+    let window = Windows.list[index]
+    guard let newTitle = window.axUiElement.title(),
           newTitle != window.title else { return }
     window.title = newTitle
-    window.refreshThumbnail()
     app.refreshOpenUi()
 }
 
-
+private func eventWindowResized(_ app: App, _ element: AXUIElement) {
+    guard let index = Windows.list.firstIndexThatMatches(element) else { return }
+    app.refreshOpenUi()
+}

src/logic/Windows.swift

@@ -3,26 +3,34 @@ import Cocoa
 class Windows {
     // order in the array is important: most-recently-used elements are first
     static var list = [Window]()
+    static var previousFocusedWindowIndex = Array<Window>.Index(0)
     static var focusedWindowIndex = Array<Window>.Index(0)
     static var windowsInSubscriptionRetryLoop = [String]()
 
+    static func updateFocusedWindowIndex(_ newValue: Array<Window>.Index) {
+        previousFocusedWindowIndex = focusedWindowIndex
+        focusedWindowIndex = newValue
+        let focusedView = ThumbnailsView.recycledViews[focusedWindowIndex]
+        ThumbnailsPanel.highlightCell(ThumbnailsView.recycledViews[previousFocusedWindowIndex], focusedView)
+        (App.shared as! App).thumbnailsPanel!.thumbnailsView.scrollView.contentView.scrollToVisible(focusedView.frame)
+    }
+
     static func focusedWindow() -> Window? {
         return list.count > focusedWindowIndex ? list[focusedWindowIndex] : nil
     }
 
     static func cycleFocusedWindowIndex(_ step: Array<Window>.Index) {
-        focusedWindowIndex = focusedWindowIndex + step < 0 ? list.count - 1 : (focusedWindowIndex + step) % list.count
+        updateFocusedWindowIndex(focusedWindowIndex + step < 0 ? list.count - 1 : (focusedWindowIndex + step) % list.count)
     }
 
     static func moveFocusedWindowIndexAfterWindowDestroyedInBackground(_ destroyedWindowIndex: Array<Window>.Index) {
         if focusedWindowIndex <= destroyedWindowIndex {
-            focusedWindowIndex -= 1
-            return
+            updateFocusedWindowIndex(max(focusedWindowIndex - 1, 0))
         }
     }
 
     static func moveFocusedWindowIndexAfterWindowCreatedInBackground() {
-        focusedWindowIndex += 1
+        updateFocusedWindowIndex(focusedWindowIndex + 1)
     }
 
     static func updateSpaces() {

src/ui/App.swift

@@ -42,6 +42,8 @@ class App: NSApplication, NSApplicationDelegate {
         Keyboard.listenToGlobalEvents(self)
         preferencesWindow = PreferencesWindow()
         UpdatesTab.observeUserDefaults()
+//        UserDefaults.standard.set(false, forKey: "NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints")
+        // TODO: add warm up code for faster first launch
     }
 
     // keyboard shortcuts are broken without a menu. We generated the default menu from XCode and load it
@@ -67,7 +69,6 @@ class App: NSApplication, NSApplicationDelegate {
     func focusTarget() {
         debugPrint("focusTarget")
         if appIsBeingUsed {
-            debugPrint("focusTarget: appIsBeingUsed")
             let window = Windows.focusedWindow()
             focusSelectedWindow(window)
         }
@@ -95,13 +96,35 @@ class App: NSApplication, NSApplicationDelegate {
 
     @objc
     func showUi() {
-        uiWorkShouldBeDone = true
-        showUiOrCycleSelection(0)
+        _ = dispatchWork { self.showUiOrCycleSelection(0) }
     }
 
     func cycleSelection(_ step: Int) {
         Windows.cycleFocusedWindowIndex(step)
-        thumbnailsPanel!.highlightCell()
+    }
+
+    func focusSelectedWindow(_ window: Window?) {
+        hideUi()
+        guard !CGWindow.isMissionControlActive() else { return }
+        window?.focus()
+    }
+
+    func reopenUi() {
+        // TODO: retest this
+        thumbnailsPanel!.orderOut(nil)
+        Windows.refreshAllThumbnails()
+        refreshOpenUi()
+        thumbnailsPanel!.show()
+    }
+
+    func refreshOpenUi() {
+        guard appIsBeingUsed else { return }
+        let currentScreen = Screen.preferred() // fix screen between steps since it could change (e.g. mouse moved to another screen)
+        guard uiWorkShouldBeDone else { return }
+        thumbnailsPanel!.thumbnailsView.updateItems(currentScreen)
+        thumbnailsPanel!.setFrame(thumbnailsPanel!.thumbnailsView.frame, display: false)
+        guard uiWorkShouldBeDone else { return }
+        Screen.repositionPanel(thumbnailsPanel!, currentScreen, .appleCentered)
     }
 
     func showUiOrCycleSelection(_ step: Int) {
@@ -119,39 +142,22 @@ class App: NSApplication, NSApplicationDelegate {
             Spaces.updateIsSingleSpace()
             // TODO: find a way to update space index when windows are moved to another space, instead of on every trigger
             Windows.updateSpaces()
-            // TODO: find a way to update thumbnails by listening to content change, instead of every trigger. Or better, switch to video
-            Windows.refreshAllThumbnails()
-            Windows.focusedWindowIndex = 0
+            Windows.updateFocusedWindowIndex(0)
             Windows.cycleFocusedWindowIndex(step)
-            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Preferences.windowDisplayDelay, execute: { [weak self] in
-                guard let self = self else { return }
+            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Preferences.windowDisplayDelay) {
+                guard self.uiWorkShouldBeDone else { return }
+                Windows.refreshAllThumbnails()
+                guard self.uiWorkShouldBeDone else { return }
                 self.refreshOpenUi()
-                if self.uiWorkShouldBeDone { self.thumbnailsPanel?.show() }
-            })
+                guard self.uiWorkShouldBeDone else { return }
+                self.thumbnailsPanel!.show()
+//                DispatchQueue.main.async {
+//                    guard self.uiWorkShouldBeDone else { return }
+//                    self.refreshThumbnails()
+//                }
+            }
         } else {
-            debugPrint("showUiOrCycleSelection: !isFirstSummon")
             cycleSelection(step)
         }
     }
-
-    func reopenUi() {
-        thumbnailsPanel!.orderOut(nil)
-        Windows.refreshAllThumbnails()
-        refreshOpenUi()
-        thumbnailsPanel!.show()
-    }
-
-    func refreshOpenUi() {
-        guard appIsBeingUsed else { return }
-        let currentScreen = Screen.preferred() // fix screen between steps since it could change (e.g. mouse moved to another screen)
-        if uiWorkShouldBeDone { thumbnailsPanel!.refreshCollectionView(currentScreen, uiWorkShouldBeDone); debugPrint("refreshCollectionView") }
-        if uiWorkShouldBeDone { thumbnailsPanel!.highlightCell(); debugPrint("highlightCellAt") }
-        if uiWorkShouldBeDone { Screen.repositionPanel(thumbnailsPanel!, currentScreen, .appleCentered); debugPrint("repositionPanel") }
-    }
-
-    func focusSelectedWindow(_ window: Window?) {
-        hideUi()
-        guard !CGWindow.isMissionControlActive() else { return }
-        window?.focus()
-    }
 }

src/ui/main-window/CollectionViewFlowLayout.swift

@@ -1,62 +0,0 @@
-import Cocoa
-
-class CollectionViewFlowLayout: NSCollectionViewFlowLayout {
-    var currentScreen: NSScreen?
-    var widestRow: CGFloat?
-    var totalHeight: CGFloat?
-
-    override func layoutAttributesForElements(in rect: CGRect) -> [NSCollectionViewLayoutAttributes] {
-        let attributes_ = super.layoutAttributesForElements(in: rect)
-        guard !attributes_.isEmpty else { return attributes_ }
-        let attributes = NSArray(array: attributes_, copyItems: true) as! [NSCollectionViewLayoutAttributes]
-        var currentRow: [NSCollectionViewLayoutAttributes] = []
-        var currentRowY = CGFloat(0)
-        var currentRowWidth = CGFloat(0)
-        var previousRowMaxHeight = CGFloat(0)
-        var currentRowMaxHeight = CGFloat(0)
-        var widestRow = CGFloat(0)
-        var totalHeight = CGFloat(0)
-        for (index, attribute) in attributes.enumerated() {
-            let isNewRow = abs(attribute.frame.origin.y - currentRowY) > CollectionViewItemView.height(currentScreen!)
-            if isNewRow {
-                currentRowWidth -= Preferences.interCellPadding
-                widestRow = max(widestRow, currentRowWidth)
-                setCenteredPositionForPreviousRowCells(currentRowWidth, previousRowMaxHeight, currentRow)
-                currentRow.removeAll()
-                currentRowY = attribute.frame.origin.y
-                currentRowWidth = 0
-                previousRowMaxHeight += currentRowMaxHeight + Preferences.interCellPadding
-                currentRowMaxHeight = 0
-            }
-            currentRow.append(attribute)
-            currentRowWidth += attribute.frame.size.width + Preferences.interCellPadding
-            currentRowMaxHeight = max(currentRowMaxHeight, attribute.frame.size.height)
-            if index == attributes.count - 1 {
-                currentRowWidth -= Preferences.interCellPadding
-                widestRow = max(widestRow, currentRowWidth)
-                totalHeight = previousRowMaxHeight + currentRowMaxHeight
-                setCenteredPositionForPreviousRowCells(currentRowWidth, previousRowMaxHeight, currentRow)
-            }
-        }
-        shiftCenteredElementToTheLeft(attributes, widestRow, totalHeight)
-        self.widestRow = widestRow
-        self.totalHeight = totalHeight
-        return attributes
-    }
-
-    private func shiftCenteredElementToTheLeft(_ attributes: [NSCollectionViewLayoutAttributes], _ widestRow: CGFloat, _ totalHeight: CGFloat) {
-        let horizontalMargin = ((collectionView!.frame.size.width - widestRow) / 2).rounded()
-        for attribute in attributes {
-            attribute.frame.origin.x -= horizontalMargin
-        }
-    }
-
-    private func setCenteredPositionForPreviousRowCells(_ currentRowWidth: CGFloat, _ previousRowMaxHeight: CGFloat, _ currentRow: [NSCollectionViewLayoutAttributes]) {
-        var marginLeft = (collectionView!.frame.size.width - currentRowWidth) / 2
-        for attribute in currentRow {
-            attribute.frame.origin.x = marginLeft
-            attribute.frame.origin.y = previousRowMaxHeight
-            marginLeft += attribute.frame.size.width + Preferences.interCellPadding
-        }
-    }
-}

src/ui/main-window/CollectionViewItem.swift

@@ -1,18 +0,0 @@
-import Cocoa
-import WebKit
-
-class CollectionViewItem: NSCollectionViewItem {
-    var view_: CollectionViewItemView { view as! CollectionViewItemView }
-
-    override func loadView() {
-        view = CollectionViewItemView()
-        view.wantsLayer = true
-    }
-
-    override var isSelected: Bool {
-        didSet {
-            view.layer!.backgroundColor = isSelected ? Preferences.highlightBackgroundColor.cgColor : .clear
-            view.layer!.borderColor = isSelected ? Preferences.highlightBorderColor.cgColor : .clear
-        }
-    }
-}

src/ui/main-window/ThumbnailFontIconView.swift

@@ -2,7 +2,7 @@ import Cocoa
 
 // Font icon using SF Symbols from the SF Pro font from Apple
 // see https://developer.apple.com/design/human-interface-guidelines/sf-symbols/overview/
-class FontIcon: CellTitle {
+class ThumbnailFontIconView: ThumbnailTitleView {
     static let sfSymbolCircledMinusSign = "􀁎"
     static let sfSymbolCircledDotSign = "􀍷"
     static let sfSymbolCircledNumber0 = "􀀸"
@@ -22,19 +22,19 @@ class FontIcon: CellTitle {
     // number should be in the interval [0-50]
     func setNumber(_ number: UInt32) {
         let (baseCharacter, offset) = baseCharacterAndOffset(number)
-        string = String(UnicodeScalar(baseCharacter.unicodeScalars.first!.value + offset)!)
+        assignIfDifferent(&string, String(UnicodeScalar(baseCharacter.unicodeScalars.first!.value + offset)!))
     }
 
     func setStar() {
-        string = FontIcon.sfSymbolCircledStart
+        assignIfDifferent(&string, ThumbnailFontIconView.sfSymbolCircledStart)
     }
 
     private func baseCharacterAndOffset(_ number: UInt32) -> (String, UInt32) {
         if number <= 9 {
             // numbers alternate between empty and full circles; we skip the full circles
-            return (FontIcon.sfSymbolCircledNumber0, number * UInt32(2))
+            return (ThumbnailFontIconView.sfSymbolCircledNumber0, number * UInt32(2))
         } else {
-            return (FontIcon.sfSymbolCircledNumber10, number - 10)
+            return (ThumbnailFontIconView.sfSymbolCircledNumber10, number - 10)
         }
     }
 }

src/ui/main-window/ThumbnailTitleView.swift

@@ -1,6 +1,6 @@
 import Cocoa
 
-class CellTitle: BaseLabel {
+class ThumbnailTitleView: BaseLabel {
     var magicOffset = CGFloat(0)
 
     convenience init(_ size: CGFloat, _ magicOffset: CGFloat = 0) {
@@ -14,7 +14,7 @@ class CellTitle: BaseLabel {
         self.init(NSRect.zero, textContainer)
         self.magicOffset = magicOffset
         textColor = Preferences.fontColor
-        shadow = CollectionViewItemView.makeShadow(.darkGray)
+        shadow = ThumbnailView.makeShadow(.darkGray)
         defaultParagraphStyle = makeParagraphStyle(size)
         heightAnchor.constraint(equalToConstant: size + magicOffset).isActive = true
     }

src/ui/main-window/ThumbnailView.swift

@@ -1,13 +1,13 @@
 import Cocoa
 
-class CollectionViewItemView: NSStackView {
+class ThumbnailView: NSStackView {
     var window_: Window?
     var thumbnail = NSImageView()
     var appIcon = NSImageView()
-    var label = CellTitle(Preferences.fontHeight)
-    var minimizedIcon = FontIcon(FontIcon.sfSymbolCircledMinusSign, Preferences.fontIconSize, .white)
-    var hiddenIcon = FontIcon(FontIcon.sfSymbolCircledDotSign, Preferences.fontIconSize, .white)
-    var spaceIcon = FontIcon(FontIcon.sfSymbolCircledNumber0, Preferences.fontIconSize, .white)
+    var label = ThumbnailTitleView(Preferences.fontHeight)
+    var minimizedIcon = ThumbnailFontIconView(ThumbnailFontIconView.sfSymbolCircledMinusSign, Preferences.fontIconSize, .white)
+    var hiddenIcon = ThumbnailFontIconView(ThumbnailFontIconView.sfSymbolCircledDotSign, Preferences.fontIconSize, .white)
+    var spaceIcon = ThumbnailFontIconView(ThumbnailFontIconView.sfSymbolCircledNumber0, Preferences.fontIconSize, .white)
     var mouseDownCallback: MouseDownCallback!
     var mouseMovedCallback: MouseMovedCallback!
     var dragAndDropTimer: Timer?
@@ -16,12 +16,58 @@ class CollectionViewItemView: NSStackView {
         self.init(frame: .zero)
         let hStackView = makeHStackView()
         setupView(hStackView)
-        let shadow = CollectionViewItemView.makeShadow(.gray)
+        let shadow = ThumbnailView.makeShadow(.gray)
         thumbnail.shadow = shadow
         appIcon.shadow = shadow
         observeDragAndDrop()
     }
 
+    func updateRecycledCellWithNewContent(_ element: Window, _ mouseDownCallback: @escaping MouseDownCallback, _ mouseMovedCallback: @escaping MouseMovedCallback, _ newHeight: CGFloat,_ screen: NSScreen) {
+        window_ = element
+//        element.itemView = self
+        if thumbnail.image != element.thumbnail {
+            thumbnail.image = element.thumbnail
+            let (thumbnailWidth, thumbnailHeight) = ThumbnailView.thumbnailSize(element.thumbnail, screen)
+            let thumbnailSize = NSSize(width: thumbnailWidth.rounded(), height: thumbnailHeight.rounded())
+            thumbnail.image?.size = thumbnailSize
+            thumbnail.frame.size = thumbnailSize
+        }
+        if appIcon.image != element.icon {
+            appIcon.image = element.icon
+            let appIconSize = NSSize(width: Preferences.iconSize, height: Preferences.iconSize)
+            appIcon.image?.size = appIconSize
+            appIcon.frame.size = appIconSize
+        }
+        if label.string != element.title {
+            label.string = element.title
+            // workaround: setting string on NSTextView change the font (most likely a Cocoa bug)
+            label.font = Preferences.font
+        }
+        assignIfDifferent(&hiddenIcon.isHidden, !window_!.isHidden)
+        assignIfDifferent(&minimizedIcon.isHidden, !window_!.isMinimized)
+        assignIfDifferent(&spaceIcon.isHidden, element.spaceIndex == nil || Spaces.isSingleSpace || Preferences.hideSpaceNumberLabels)
+        if !spaceIcon.isHidden {
+            if element.isOnAllSpaces {
+                spaceIcon.setStar()
+            } else {
+                spaceIcon.setNumber(UInt32(element.spaceIndex!))
+            }
+        }
+        assignIfDifferent(&frame.size.width, thumbnail.frame.size.width + Preferences.intraCellPadding * 2)
+        assignIfDifferent(&frame.size.height, newHeight)
+        let fontIconWidth = CGFloat([minimizedIcon, hiddenIcon, spaceIcon].filter { !$0.isHidden }.count) * (Preferences.fontIconSize + Preferences.intraCellPadding)
+        assignIfDifferent(&label.textContainer!.size.width, frame.width - Preferences.iconSize - Preferences.intraCellPadding * 3 - fontIconWidth)
+        assignIfDifferent(&subviews.first!.frame.size, frame.size)
+        self.mouseDownCallback = mouseDownCallback
+        self.mouseMovedCallback = mouseMovedCallback
+        if trackingAreas.count == 0 {
+            addTrackingArea(NSTrackingArea(rect: bounds, options: [.mouseMoved, .activeAlways], owner: self, userInfo: nil))
+        } else if trackingAreas.count > 0 && trackingAreas[0].rect != bounds {
+            removeTrackingArea(trackingAreas[0])
+            addTrackingArea(NSTrackingArea(rect: bounds, options: [.mouseMoved, .activeAlways], owner: self, userInfo: nil))
+        }
+    }
+
     private func observeDragAndDrop() {
         // NSImageView instances are registered to drag-and-drop by default
         thumbnail.unregisterDraggedTypes()
@@ -63,41 +109,6 @@ class CollectionViewItemView: NSStackView {
         mouseDownCallback()
     }
 
-    func updateRecycledCellWithNewContent(_ element: Window, _ mouseDownCallback: @escaping MouseDownCallback, _ mouseMovedCallback: @escaping MouseMovedCallback, _ screen: NSScreen) {
-        window_ = element
-        thumbnail.image = element.thumbnail
-        let (thumbnailWidth, thumbnailHeight) = CollectionViewItemView.thumbnailSize(element.thumbnail, screen)
-        let thumbnailSize = NSSize(width: thumbnailWidth.rounded(), height: thumbnailHeight.rounded())
-        thumbnail.image?.size = thumbnailSize
-        thumbnail.frame.size = thumbnailSize
-        appIcon.image = element.icon
-        let appIconSize = NSSize(width: Preferences.iconSize, height: Preferences.iconSize)
-        appIcon.image?.size = appIconSize
-        appIcon.frame.size = appIconSize
-        label.string = element.title
-        // workaround: setting string on NSTextView change the font (most likely a Cocoa bug)
-        label.font = Preferences.font
-        hiddenIcon.isHidden = !window_!.isHidden
-        minimizedIcon.isHidden = !window_!.isMinimized
-        spaceIcon.isHidden = element.spaceIndex == nil || Spaces.isSingleSpace || Preferences.hideSpaceNumberLabels
-        if !spaceIcon.isHidden {
-            if element.isOnAllSpaces {
-                spaceIcon.setStar()
-            } else {
-                spaceIcon.setNumber(UInt32(element.spaceIndex!))
-            }
-        }
-        let fontIconWidth = CGFloat([minimizedIcon, hiddenIcon, spaceIcon].filter { !$0.isHidden }.count) * (Preferences.fontIconSize + Preferences.intraCellPadding)
-        label.textContainer!.size.width = frame.width - Preferences.iconSize - Preferences.intraCellPadding * 3 - fontIconWidth
-        subviews.first!.frame.size = frame.size
-        self.mouseDownCallback = mouseDownCallback
-        self.mouseMovedCallback = mouseMovedCallback
-        if trackingAreas.count > 0 {
-            removeTrackingArea(trackingAreas[0])
-        }
-        addTrackingArea(NSTrackingArea(rect: bounds, options: [.mouseMoved, .activeAlways], owner: self, userInfo: nil))
-    }
-
     static func makeShadow(_ color: NSColor) -> NSShadow {
         let shadow = NSShadow()
         shadow.shadowColor = color
@@ -106,33 +117,26 @@ class CollectionViewItemView: NSStackView {
         return shadow
     }
 
-    static func downscaleFactor() -> CGFloat {
-        let nCellsBeforePotentialOverflow = Preferences.minRows * Preferences.minCellsPerRow
-        guard CGFloat(Windows.list.count) > nCellsBeforePotentialOverflow else { return 1 }
-        // TODO: replace this buggy heuristic with a correct implementation of downscaling
-        return nCellsBeforePotentialOverflow / (nCellsBeforePotentialOverflow + (sqrt(CGFloat(Windows.list.count) - nCellsBeforePotentialOverflow) * 2))
-    }
-
     static func widthMax(_ screen: NSScreen) -> CGFloat {
-        return (ThumbnailsPanel.widthMax(screen) / Preferences.minCellsPerRow - Preferences.interCellPadding) * CollectionViewItemView.downscaleFactor()
+        return ThumbnailsPanel.widthMax(screen) / Preferences.minCellsPerRow - Preferences.interCellPadding
     }
 
     static func widthMin(_ screen: NSScreen) -> CGFloat {
-        return (ThumbnailsPanel.widthMax(screen) / Preferences.maxCellsPerRow - Preferences.interCellPadding) * CollectionViewItemView.downscaleFactor()
+        return ThumbnailsPanel.widthMax(screen) / Preferences.maxCellsPerRow - Preferences.interCellPadding
     }
 
     static func height(_ screen: NSScreen) -> CGFloat {
-        return (ThumbnailsPanel.heightMax(screen) / Preferences.minRows - Preferences.interCellPadding) * CollectionViewItemView.downscaleFactor()
+        return ThumbnailsPanel.heightMax(screen) / Preferences.rowsCount - Preferences.interCellPadding
     }
 
     static func width(_ image: NSImage?, _ screen: NSScreen) -> CGFloat {
-        return max(thumbnailSize(image, screen).0 + Preferences.intraCellPadding * 2, CollectionViewItemView.widthMin(screen))
+        return max(thumbnailSize(image, screen).0 + Preferences.intraCellPadding * 2, ThumbnailView.widthMin(screen))
     }
 
     static func thumbnailSize(_ image: NSImage?, _ screen: NSScreen) -> (CGFloat, CGFloat) {
         guard let image = image else { return (0, 0) }
-        let thumbnailHeightMax = CollectionViewItemView.height(screen) - Preferences.intraCellPadding * 3 - Preferences.iconSize
-        let thumbnailWidthMax = CollectionViewItemView.widthMax(screen) - Preferences.intraCellPadding * 2
+        let thumbnailHeightMax = ThumbnailView.height(screen) - Preferences.intraCellPadding * 3 - Preferences.iconSize
+        let thumbnailWidthMax = ThumbnailView.widthMax(screen) - Preferences.intraCellPadding * 2
         let thumbnailHeight = min(image.size.height, thumbnailHeightMax)
         let thumbnailWidth = min(image.size.width, thumbnailWidthMax)
         let imageRatio = image.size.width / image.size.height

src/ui/main-window/ThumbnailsPanel.swift

@@ -1,15 +1,12 @@
 import Cocoa
 
-class ThumbnailsPanel: NSPanel, NSCollectionViewDataSource, NSCollectionViewDelegate, NSCollectionViewDelegateFlowLayout {
-    var backgroundView: NSVisualEffectView!
-    var collectionView: NSCollectionView!
-    var app: App?
+class ThumbnailsPanel: NSPanel {
+    var thumbnailsView = ThumbnailsView()
     var currentScreen: NSScreen?
     static let cellId = NSUserInterfaceItemIdentifier("Cell")
 
     convenience init(_ app: App) {
-        self.init()
-        self.app = app
+        self.init(contentRect: .zero, styleMask: .utilityWindow, backing: .buffered, defer: true)
         isFloatingPanel = true
         animationBehavior = .none
         hidesOnDeactivate = false
@@ -17,10 +14,7 @@ class ThumbnailsPanel: NSPanel, NSCollectionViewDataSource, NSCollectionViewDele
         titleVisibility = .hidden
         styleMask.remove(.titled)
         backgroundColor = .clear
-        makeCollectionView()
-        backgroundView = ThumbnailsPanel.makeBackgroundView()
-        backgroundView.addSubview(collectionView)
-        contentView!.addSubview(backgroundView)
+        contentView!.addSubview(thumbnailsView)
         // 2nd highest level possible; this allows the app to go on top of context menus
         // highest level is .screenSaver but makes drag and drop on top the main window impossible
         level = .popUpMenu
@@ -32,79 +26,11 @@ class ThumbnailsPanel: NSPanel, NSCollectionViewDataSource, NSCollectionViewDele
         makeKeyAndOrderFront(nil)
     }
 
-    static func makeBackgroundView() -> NSVisualEffectView {
-        let backgroundView = NSVisualEffectView()
-        backgroundView.translatesAutoresizingMaskIntoConstraints = false
-        backgroundView.material = Preferences.windowMaterial
-        backgroundView.state = .active
-        backgroundView.wantsLayer = true
-        backgroundView.layer!.cornerRadius = Preferences.windowCornerRadius
-        return backgroundView
-    }
-
-    func makeCollectionView() {
-        collectionView = NSCollectionView()
-        collectionView.dataSource = self
-        collectionView.delegate = self
-        collectionView.collectionViewLayout = makeLayout()
-        collectionView.backgroundColors = [.clear]
-        collectionView.isSelectable = true
-        collectionView.allowsMultipleSelection = false
-        collectionView.register(CollectionViewItem.self, forItemWithIdentifier: ThumbnailsPanel.cellId)
-    }
-
-    private func makeLayout() -> CollectionViewFlowLayout {
-        let layout = CollectionViewFlowLayout()
-        layout.minimumInteritemSpacing = Preferences.interCellPadding
-        layout.minimumLineSpacing = Preferences.interCellPadding
-        return layout
-    }
-
-    func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
-        return Windows.list.count
-    }
-
-    func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
-        let item = collectionView.makeItem(withIdentifier: ThumbnailsPanel.cellId, for: indexPath) as! CollectionViewItem
-        item.view_.updateRecycledCellWithNewContent(Windows.list[indexPath.item],
-                { self.app!.focusSelectedWindow(item.view_.window_) },
-                { self.app!.thumbnailsPanel!.highlightCell(item) },
-                currentScreen!)
-        return item
-    }
-
-    func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> NSSize {
-        guard indexPath.item < Windows.list.count else { return .zero }
-        return NSSize(width: CollectionViewItemView.width(Windows.list[indexPath.item].thumbnail, currentScreen!).rounded(), height: CollectionViewItemView.height(currentScreen!).rounded())
-    }
-
-    func highlightCell() {
-        collectionView.deselectAll(nil)
-        collectionView.selectItems(at: [IndexPath(item: Windows.focusedWindowIndex, section: 0)], scrollPosition: .top)
-    }
-
-    func highlightCell(_ cell: CollectionViewItem) {
-        let newIndex = collectionView.indexPath(for: cell)!
-        if Windows.focusedWindowIndex != newIndex.item {
-            collectionView.selectItems(at: [newIndex], scrollPosition: .top)
-            collectionView.deselectItems(at: [IndexPath(item: Windows.focusedWindowIndex, section: 0)])
-            Windows.focusedWindowIndex = newIndex.item
-        }
-    }
-
-    func refreshCollectionView(_ screen: NSScreen, _ uiWorkShouldBeDone: Bool) {
-        if uiWorkShouldBeDone { self.currentScreen = screen }
-        let layout = collectionView.collectionViewLayout as! CollectionViewFlowLayout
-        if uiWorkShouldBeDone { layout.currentScreen = screen }
-        if uiWorkShouldBeDone { layout.invalidateLayout() }
-        if uiWorkShouldBeDone { collectionView.setFrameSize(NSSize(width: ThumbnailsPanel.widthMax(screen).rounded(), height: ThumbnailsPanel.heightMax(screen).rounded())) }
-        if uiWorkShouldBeDone { collectionView.reloadData() }
-        if uiWorkShouldBeDone { collectionView.layoutSubtreeIfNeeded() }
-        if uiWorkShouldBeDone { collectionView.setFrameSize(NSSize(width: layout.widestRow!, height: layout.totalHeight!)) }
-        let windowSize = NSSize(width: layout.widestRow! + Preferences.windowPadding * 2, height: layout.totalHeight! + Preferences.windowPadding * 2)
-        if uiWorkShouldBeDone { setContentSize(windowSize) }
-        if uiWorkShouldBeDone { backgroundView!.setFrameSize(windowSize) }
-        if uiWorkShouldBeDone { collectionView.setFrameOrigin(NSPoint(x: Preferences.windowPadding, y: Preferences.windowPadding)) }
+    static func highlightCell(_ previousView: NSView, _ newView: NSView) {
+        previousView.layer!.backgroundColor = .clear
+        previousView.layer!.borderColor = .clear
+        newView.layer!.backgroundColor = Preferences.highlightBackgroundColor.cgColor
+        newView.layer!.borderColor = Preferences.highlightBorderColor.cgColor
     }
 
     static func widthMax(_ screen: NSScreen) -> CGFloat {

src/ui/main-window/ThumbnailsView.swift

@@ -0,0 +1,104 @@
+import Cocoa
+
+class ThumbnailsView: NSVisualEffectView {
+    let scrollView = ScrollView()
+    static var recycledViews = [ThumbnailView]()
+
+    convenience init() {
+        self.init(frame: .zero)
+        material = Preferences.windowMaterial
+        state = .active
+        wantsLayer = true
+        layer!.cornerRadius = Preferences.windowCornerRadius
+        addSubview(scrollView)
+        // TODO: think about this optimization more
+        (1...100).forEach { _ in ThumbnailsView.recycledViews.append(ThumbnailView()) }
+    }
+
+    func updateItems(_ screen: NSScreen) {
+        let widthMax = ThumbnailsPanel.widthMax(screen).rounded()
+        let heightMax = ThumbnailsPanel.heightMax(screen).rounded()
+        let height = ThumbnailView.height(screen).rounded(.down)
+        var currentX = CGFloat(0)
+        var currentY = CGFloat(0)
+        var maxX = CGFloat(0)
+        var maxY = height
+        var newViews = [ThumbnailView]()
+        for (index, window) in Windows.list.enumerated() {
+            let view = ThumbnailsView.recycledViews[index]
+            view.updateRecycledCellWithNewContent(window,
+                    { (App.shared as! App).focusSelectedWindow(window) },
+                    { Windows.updateFocusedWindowIndex(index) },
+                    height, screen)
+            let width = view.frame.size.width
+            if (currentX + Preferences.interCellPadding + width).rounded(.down) > widthMax {
+                currentX = CGFloat(0)
+                currentY = (currentY + Preferences.interCellPadding + height).rounded(.down)
+                maxY = max(currentY + height, maxY)
+            } else {
+                maxX = max(currentX + width, maxX)
+            }
+            view.frame.origin = CGPoint(x: currentX, y: currentY)
+            currentX = (currentX + Preferences.interCellPadding + width).rounded(.down)
+            newViews.append(view)
+        }
+        scrollView.documentView!.subviews = newViews
+        frame.size = NSSize(width: min(maxX, widthMax) + Preferences.windowPadding * 2, height: min(maxY, heightMax) + Preferences.windowPadding * 2)
+        scrollView.frame.size = NSSize(width: min(maxX, widthMax), height: min(maxY, heightMax))
+        scrollView.frame.origin = CGPoint(x: Preferences.windowPadding, y: Preferences.windowPadding)
+        scrollView.contentView.frame.size = scrollView.frame.size
+        scrollView.documentView!.frame.size = NSSize(width: maxX, height: maxY)
+        if Preferences.alignThumbnails == .center {
+            centerRows(maxX)
+        }
+    }
+
+    func centerRows(_ maxX: CGFloat) {
+        var rowStartIndex = 0
+        var rowWidth = CGFloat(0)
+        var rowY = CGFloat(0)
+        for (index, window) in Windows.list.enumerated() {
+            let view = ThumbnailsView.recycledViews[index]
+            if view.frame.origin.y == rowY {
+                rowWidth += Preferences.interCellPadding + view.frame.size.width
+            } else {
+                if rowStartIndex == 0 {
+                    rowWidth -= Preferences.interCellPadding // first row has 1 extra padding
+                }
+                shiftRow(maxX, rowWidth, rowStartIndex, index)
+                rowStartIndex = index
+                rowWidth = view.frame.size.width
+                rowY = view.frame.origin.y
+            }
+        }
+        shiftRow(maxX, rowWidth, rowStartIndex, Windows.list.count)
+    }
+
+    private func shiftRow(_ maxX: CGFloat, _ rowWidth: CGFloat, _ rowStartIndex: Int, _ index: Int) {
+        let offset = ((maxX - rowWidth) / 2).rounded()
+        if offset > 0 {
+            (rowStartIndex..<index).forEach {
+                ThumbnailsView.recycledViews[$0].frame.origin.x += offset
+            }
+        }
+    }
+}
+
+class ScrollView: NSScrollView {
+    convenience init() {
+        self.init(frame: .zero)
+        documentView = FlippedView(frame: .zero)
+        drawsBackground = false
+        hasVerticalScroller = true
+    }
+
+    override func tile() {
+        super.tile()
+        // draw the scroller on top of the content
+        contentView.frame = bounds
+    }
+}
+
+class FlippedView: NSView {
+    override var isFlipped: Bool { true }
+}

src/ui/preferences-window/tabs/AppearanceTab.swift

@@ -10,10 +10,11 @@ class AppearanceTab {
     private static func makeView() -> NSGridView {
         let view = GridView.make([
             LabelAndControl.makeLabelWithDropdown(NSLocalizedString("Theme", comment: ""), "theme", MacroPreferences.themeList.values.map { $0.label }),
+            LabelAndControl.makeLabelWithDropdown(NSLocalizedString("Align windows", comment: ""), "alignThumbnails", Array(MacroPreferences.alignThumbnailsList.keys)),
             LabelAndControl.makeLabelWithSlider(NSLocalizedString("Max size on screen", comment: ""), "maxScreenUsage", 10, 100, 10, true, "%"),
             LabelAndControl.makeLabelWithSlider(NSLocalizedString("Min windows per row", comment: ""), "minCellsPerRow", 1, 20, 20, true),
             LabelAndControl.makeLabelWithSlider(NSLocalizedString("Max windows per row", comment: ""), "maxCellsPerRow", 1, 40, 20, true),
-            LabelAndControl.makeLabelWithSlider(NSLocalizedString("Min rows of windows", comment: ""), "minRows", 1, 20, 20, true),
+            LabelAndControl.makeLabelWithSlider(NSLocalizedString("Rows of windows", comment: ""), "rowsCount", 1, 20, 20, true),
             LabelAndControl.makeLabelWithSlider(NSLocalizedString("Window app icon size", comment: ""), "iconSize", 0, 64, 11, false, "px"),
             LabelAndControl.makeLabelWithSlider(NSLocalizedString("Window title font size", comment: ""), "fontHeight", 0, 64, 11, false, "px"),
             LabelAndControl.makeLabelWithDropdown(NSLocalizedString("Show on", comment: ""), "showOnScreen", Array(MacroPreferences.showOnScreenList.keys)),

src/ui/preferences-window/tabs/GeneralTab.swift

@@ -50,7 +50,7 @@ class GeneralTab {
             LSSharedFileListInsertItemURL(loginItems, kLSSharedFileListItemBeforeFirst.takeRetainedValue(), nil, nil, App.url, nil, nil)
         } else {
             loginItemsSnapshot.forEach {
-                if CFEqual(LSSharedFileListItemCopyResolvedURL($0, 0, nil).takeRetainedValue(), App.url) {
+                if LSSharedFileListItemCopyResolvedURL($0, 0, nil).takeRetainedValue() == App.url {
                     LSSharedFileListItemRemove(loginItems, $0)
                 }
             }