fix: various issues with the app freezing or being slowly

src/logic/Application.swift

@@ -54,7 +54,7 @@ class Application: NSObject {
     func removeWindowslessAppWindow() {
         if let windowlessAppWindow = (Windows.list.firstIndex { $0.isWindowlessApp == true && $0.application.pid == pid }) {
             Windows.list.remove(at: windowlessAppWindow)
-            App.app.refreshOpenUi([])
+            App.app.refreshOpenUi([], .refreshUiAfterExternalEvent)
         }
     }
 
@@ -72,9 +72,11 @@ class Application: NSObject {
         retryAxCallUntilTimeout(group, 5) { [weak self] in
             guard let self = self else { return }
             var atLeastOneActualWindow = false
-            if let axWindows_ = try self.axUiElement!.windows(), axWindows_.count > 0 {
+            if let axWindows_ = try self.axUiElement!.windows(), !axWindows_.isEmpty {
                 // bug in macOS: sometimes the OS returns multiple duplicate windows (e.g. Mail.app starting at login)
-                try Array(Set(axWindows_)).forEach { axWindow in
+                let uniqueWindows = Array(Set(axWindows_))
+                if uniqueWindows.isEmpty { return }
+                for axWindow in uniqueWindows {
                     if let wid = try axWindow.cgWindowId() {
                         let title = try axWindow.title()
                         let subrole = try axWindow.subrole()
@@ -96,7 +98,7 @@ class Application: NSObject {
                                     window.position = position
                                 } else {
                                     let window = self.addWindow(axWindow, wid, title, isFullscreen, isMinimized, position, size)
-                                    App.app.refreshOpenUi([window])
+                                    App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
                                 }
                             }
                         }
@@ -107,7 +109,7 @@ class Application: NSObject {
                 DispatchQueue.main.async { [weak self] in
                     guard let self = self else { return }
                     if self.addWindowlessWindowIfNeeded() != nil {
-                        App.app.refreshOpenUi([])
+                        App.app.refreshOpenUi([], .refreshUiAfterExternalEvent)
                     }
                 }
                 // workaround: some apps launch but take a while to create their window(s)

src/logic/Applications.swift

@@ -83,7 +83,7 @@ class Applications {
                 Windows.cycleFocusedWindowIndex(-windowsOnTheLeftOfFocusedWindow)
             }
             if !existingWindowstoRemove.isEmpty {
-                App.app.refreshOpenUi([])
+                App.app.refreshOpenUi([], .refreshUiAfterExternalEvent)
             }
         }
     }

src/logic/BackgroundWork.swift

@@ -12,16 +12,18 @@ class BackgroundWork {
     static var repeatingKeyThread: BackgroundThreadWithRunLoop!
     static var missionControlThread: BackgroundThreadWithRunLoop!
     static var cliEventsThread: BackgroundThreadWithRunLoop!
-    static let screenshotsDispatchGroup = DispatchGroup()
 
     // swift static variables are lazy; we artificially force the threads to init
     static func start() {
-        // screenshots are taken off the main thread, concurrently
-        screenshotsQueue = DispatchQueue.globalConcurrent("screenshotsQueue", .userInteractive)
+        // screenshots are taken on a serial DispatchQueue. They used to be taken on the .global() concurrent queue.
+        // it could hand the app the screenshot OS calls are slow. It would hang or crash with this error:
+        // >Processes reached dispatch thread soft limit (64)
+        screenshotsQueue = DispatchQueue.queue("screenshotsQueue", .userInteractive, false)
         // calls to act on windows (e.g. AXUIElementSetAttributeValue, AXUIElementPerformAction) are done off the main thread
-        accessibilityCommandsQueue = DispatchQueue.globalConcurrent("accessibilityCommandsQueue", .userInteractive)
-        // calls to the AX APIs are blocking. We dispatch those on a globalConcurrent queue
-        axCallsQueue = DispatchQueue.globalConcurrent("axCallsQueue", .userInteractive)
+        accessibilityCommandsQueue = DispatchQueue.queue("accessibilityCommandsQueue", .userInteractive, false)
+        // calls to the AX APIs can block for a long time (e.g. if an app is unresponsive)
+        // We can't use a serial queue. We use the global concurrent queue
+        axCallsQueue = DispatchQueue.queue("axCallsQueue", .userInteractive, true)
         // we observe app and windows notifications. They arrive on this thread, and are handled off the main thread initially
         accessibilityEventsThread = BackgroundThreadWithRunLoop("accessibilityEventsThread", .userInteractive)
         // we listen to as any keyboard events as possible on a background thread, as it's more available/reliable than the main thread
@@ -37,7 +39,7 @@ class BackgroundWork {
     static func startCrashReportsQueue() {
         if crashReportsQueue == nil {
             // crash reports can be sent off the main thread
-            crashReportsQueue = DispatchQueue.globalConcurrent("crashReportsQueue", .utility)
+            crashReportsQueue = DispatchQueue.queue("crashReportsQueue", .utility, false)
         }
     }
 
@@ -79,27 +81,14 @@ class BackgroundWork {
     }
 }
 
-// we cap concurrent tasks to .processorCount to avoid thread explosion on the .global queue
-let backgroundWorkGlobalSemaphore = DispatchSemaphore(value: ProcessInfo.processInfo.processorCount)
-
 extension DispatchQueue {
-    static func globalConcurrent(_ label: String, _ qos: DispatchQoS) -> DispatchQueue {
-        // label is not reflected in Instruments because the target is .global
-        // if we want to see our custom labels, we need our private queue.
-        // However, we want to be efficient and use the OS thread pool, so we use .global
-        DispatchQueue(label: label, attributes: .concurrent, target: .global(qos: qos.qosClass))
-    }
-
-    func asyncWithCap(_ deadline: DispatchTime? = nil, _ fn: @escaping () -> Void) {
-        let block = {
-            fn()
-            backgroundWorkGlobalSemaphore.signal()
-        }
-        backgroundWorkGlobalSemaphore.wait()
-        if let deadline = deadline {
-            asyncAfter(deadline: deadline, execute: block)
-        } else {
-            async(execute: block)
+    static func queue(_ label: String, _ qos: DispatchQoS, _ globalParallel: Bool) -> DispatchQueue {
+        if globalParallel {
+            // label is not reflected in Instruments because the target is .global
+            // if we want to see our custom labels, we need our private queue.
+            // However, we want to be efficient and use the OS thread pool, so we use .global
+            return DispatchQueue(label: label, attributes: [.concurrent], target: .global(qos: qos.qosClass))
         }
+        return DispatchQueue(label: label, qos: qos)
     }
 }

src/logic/Window.swift

@@ -97,18 +97,17 @@ class Window {
         CFRunLoopAddSource(BackgroundWork.accessibilityEventsThread.runLoop, AXObserverGetRunLoopSource(axObserver), .commonModes)
     }
 
-    func refreshThumbnail(_ screenshot: NSImage?) {
+    func refreshThumbnail(_ screenshot: NSImage) {
         thumbnail = screenshot
-        thumbnailFullSize = screenshot?.size
-        if App.app.appIsBeingUsed && shouldShowTheUser {
-            if let index = (Windows.list.firstIndex { $0.cgWindowId == cgWindowId }) {
-                let view = ThumbnailsView.recycledViews[index]
-                if !view.thumbnail.isHidden {
-                    view.thumbnail.image = thumbnail?.copyToSeparateContexts()
-                    let thumbnailSize = ThumbnailView.thumbnailSize(thumbnail, false)
-                    view.thumbnail.setSize(thumbnailSize)
-                }
+        thumbnailFullSize = screenshot.size
+        if !App.app.appIsBeingUsed || !shouldShowTheUser { return }
+        if let view = (ThumbnailsView.recycledViews.first { $0.window_?.cgWindowId == cgWindowId }) {
+            if !view.thumbnail.isHidden {
+                view.thumbnail.image = thumbnail?.copyToSeparateContexts()
+                let thumbnailSize = ThumbnailView.thumbnailSize(thumbnail, false)
+                view.thumbnail.setSize(thumbnailSize)
             }
+            App.app.previewPanel.updateImageIfShowing(cgWindowId, screenshot, screenshot.size)
         }
     }
 
@@ -121,7 +120,7 @@ class Window {
             NSSound.beep()
             return
         }
-        BackgroundWork.accessibilityCommandsQueue.asyncWithCap { [weak self] in
+        BackgroundWork.accessibilityCommandsQueue.async { [weak self] in
             guard let self = self else { return }
             if self.isFullscreen {
                 self.axUiElement.setAttribute(kAXFullscreenAttribute, false)
@@ -141,12 +140,12 @@ class Window {
             NSSound.beep()
             return
         }
-        BackgroundWork.accessibilityCommandsQueue.asyncWithCap { [weak self] in
+        BackgroundWork.accessibilityCommandsQueue.async { [weak self] in
             guard let self = self else { return }
             if self.isFullscreen {
                 self.axUiElement.setAttribute(kAXFullscreenAttribute, false)
                 // minimizing is ignored if sent immediatly; we wait for the de-fullscreen animation to be over
-                BackgroundWork.accessibilityCommandsQueue.asyncWithCap(.now() + .seconds(1)) { [weak self] in
+                BackgroundWork.accessibilityCommandsQueue.asyncAfter(deadline: .now() + .seconds(1)) { [weak self] in
                     guard let self = self else { return }
                     self.axUiElement.setAttribute(kAXMinimizedAttribute, true)
                 }
@@ -161,7 +160,7 @@ class Window {
             NSSound.beep()
             return
         }
-        BackgroundWork.accessibilityCommandsQueue.asyncWithCap { [weak self] in
+        BackgroundWork.accessibilityCommandsQueue.async { [weak self] in
             guard let self = self else { return }
             self.axUiElement.setAttribute(kAXFullscreenAttribute, !self.isFullscreen)
         }
@@ -186,7 +185,7 @@ class Window {
             // macOS bug: when switching to a System Preferences window in another space, it switches to that space,
             // but quickly switches back to another window in that space
             // You can reproduce this buggy behaviour by clicking on the dock icon, proving it's an OS bug
-            BackgroundWork.accessibilityCommandsQueue.asyncWithCap { [weak self] in
+            BackgroundWork.accessibilityCommandsQueue.async { [weak self] in
                 guard let self = self else { return }
                 var psn = ProcessSerialNumber()
                 GetProcessForPID(self.application.pid, &psn)

src/logic/Windows.swift

@@ -273,7 +273,6 @@ class Windows {
         lazy var cgsWindowIds = Spaces.windowsInSpaces(spaceIdsAndIndexes)
         lazy var visibleCgsWindowIds = Spaces.windowsInSpaces(spaceIdsAndIndexes, false)
         for window in list {
-            // TODO: can the CGS call inside detectTabbedWindows introduce latency when WindowServer is busy?
             detectTabbedWindows(window, cgsWindowIds, visibleCgsWindowIds)
             updatesWindowSpace(window)
             refreshIfWindowShouldBeShownToTheUser(window)
@@ -303,7 +302,7 @@ class Windows {
     }
 
     // dispatch screenshot requests off the main-thread, then wait for completion
-    static func refreshThumbnails(_ windows: [Window], _ onlyUpdateScreenshots: Bool) {
+    static func refreshThumbnails(_ windows: [Window], _ source: RefreshCausedBy) {
         var eligibleWindows = [Window]()
         for window in windows {
             if !window.isWindowlessApp, let cgWindowId = window.cgWindowId, cgWindowId != CGWindowID(bitPattern: -1) {
@@ -311,30 +310,22 @@ class Windows {
             }
         }
         if eligibleWindows.isEmpty { return }
-        screenshotEligibleWindowsAndRefreshUi(eligibleWindows, onlyUpdateScreenshots)
+        screenshotEligibleWindowsAndRefreshUi(eligibleWindows, source)
     }
 
-    private static func screenshotEligibleWindowsAndRefreshUi(_ eligibleWindows: [Window], _ onlyUpdateScreenshots: Bool) {
-        eligibleWindows.forEach { _ in BackgroundWork.screenshotsDispatchGroup.enter() }
+    private static func screenshotEligibleWindowsAndRefreshUi(_ eligibleWindows: [Window], _ source: RefreshCausedBy) {
         for window in eligibleWindows {
             BackgroundWork.screenshotsQueue.async { [weak window] in
-                backgroundWorkGlobalSemaphore.wait()
-                defer {
-                    backgroundWorkGlobalSemaphore.signal()
-                    BackgroundWork.screenshotsDispatchGroup.leave()
-                }
-                if let cgImage = window?.cgWindowId?.screenshot() {
+                if source == .refreshOnlyThumbnailsAfterShowUi && !App.app.appIsBeingUsed { return }
+                if let wid = window?.cgWindowId, let cgImage = wid.screenshot() {
+                    if source == .refreshOnlyThumbnailsAfterShowUi && !App.app.appIsBeingUsed { return }
                     DispatchQueue.main.async { [weak window] in
+                        if source == .refreshOnlyThumbnailsAfterShowUi && !App.app.appIsBeingUsed { return }
                         window?.refreshThumbnail(NSImage.fromCgImage(cgImage))
                     }
                 }
             }
         }
-        if !onlyUpdateScreenshots {
-            BackgroundWork.screenshotsDispatchGroup.notify(queue: DispatchQueue.main) {
-                App.app.refreshOpenUi([])
-            }
-        }
     }
 
     static func refreshWhichWindowsToShowTheUser() {

src/logic/events/AccessibilityEvents.swift

@@ -42,7 +42,7 @@ fileprivate func applicationActivated(_ element: AXUIElement, _ pid: pid_t) thro
             let window = (appFocusedWindow != nil && wid != nil) ? Windows.updateLastFocus(appFocusedWindow!, wid!)?.first : nil
             app.focusedWindow = window
             App.app.checkIfShortcutsShouldBeDisabled(window, app.runningApplication)
-            App.app.refreshOpenUi(window != nil ? [window!] : [])
+            App.app.refreshOpenUi(window != nil ? [window!] : [], .refreshUiAfterExternalEvent)
         }
     }
 }
@@ -55,7 +55,11 @@ fileprivate func applicationHiddenOrShown(_ pid: pid_t, _ type: String) throws {
                 // for AXUIElement of apps, CFEqual or == don't work; looks like a Cocoa bug
                 return $0.application.pid == pid
             }
-            App.app.refreshOpenUi(windows)
+            // if we process the "shown" event too fast, the window won't be listed by CGSCopyWindowsWithOptionsAndTags
+            // it will thus be detected as isTabbed. We add a delay to work around this scenario
+            DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(200)) {
+                App.app.refreshOpenUi(windows, .refreshUiAfterExternalEvent)
+            }
         }
     }
 }
@@ -77,7 +81,7 @@ fileprivate func windowCreated(_ element: AXUIElement, _ pid: pid_t) throws {
                     let window = Window(element, app, wid, axTitle, isFullscreen, isMinimized, position, size)
                     Windows.appendAndUpdateFocus(window)
                     Windows.cycleFocusedWindowIndex(1)
-                    App.app.refreshOpenUi([window])
+                    App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
                 }
             }
         }
@@ -106,11 +110,11 @@ fileprivate func focusedWindowChanged(_ element: AXUIElement, _ pid: pid_t) thro
                     app.focusedWindow = w
                 }
                 if let windows = Windows.updateLastFocus(element, wid) {
-                    App.app.refreshOpenUi(windows)
+                    App.app.refreshOpenUi(windows, .refreshUiAfterExternalEvent)
                 } else if AXUIElement.isActualWindow(app, wid, level, axTitle, subrole, role, size) {
                     let window = Window(element, app, wid, axTitle, isFullscreen, isMinimized, position, size)
                     Windows.appendAndUpdateFocus(window)
-                    App.app.refreshOpenUi([window])
+                    App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
                 }
             }
         }
@@ -142,7 +146,7 @@ fileprivate func windowDestroyed(_ element: AXUIElement, _ pid: pid_t) throws {
             }
             if Windows.list.count > 0 {
                 Windows.moveFocusedWindowIndexAfterWindowDestroyedInBackground(index)
-                App.app.refreshOpenUi([])
+                App.app.refreshOpenUi([], .refreshUiAfterExternalEvent)
             } else {
                 App.app.hideUi()
             }
@@ -155,7 +159,7 @@ fileprivate func windowMiniaturizedOrDeminiaturized(_ element: AXUIElement, _ ty
         DispatchQueue.main.async {
             if let window = (Windows.list.first { $0.isEqualRobust(element, wid) }) {
                 window.isMinimized = type == kAXWindowMiniaturizedNotification
-                App.app.refreshOpenUi([window])
+                App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
             }
         }
     }
@@ -167,7 +171,7 @@ fileprivate func windowTitleChanged(_ element: AXUIElement) throws {
         DispatchQueue.main.async {
             if let window = (Windows.list.first { $0.isEqualRobust(element, wid) }), newTitle != window.title {
                 window.title = window.bestEffortTitle(newTitle)
-                App.app.refreshOpenUi([window])
+                App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
             }
         }
     }
@@ -188,7 +192,7 @@ fileprivate func windowResizedOrMoved(_ element: AXUIElement) throws {
                     window.isFullscreen = isFullscreen
                     App.app.checkIfShortcutsShouldBeDisabled(window, nil)
                 }
-                App.app.refreshOpenUi([window])
+                App.app.refreshOpenUi([window], .refreshUiAfterExternalEvent)
             }
         }
     }

src/logic/events/ScreensEvents.swift

@@ -8,7 +8,7 @@ class ScreensEvents {
     @objc private static func handleEvent(_ notification: Notification) {
         Logger.debug(notification.name.rawValue)
         // a screen added or removed can shuffle windows around Spaces; we refresh them
-        App.app.refreshOpenUi(Windows.list)
+        App.app.refreshOpenUi(Windows.list, .refreshUiAfterExternalEvent)
         Logger.info("screens", NSScreen.screens.map { ($0.uuid() ?? "nil" as CFString, $0.frame) })
         Logger.info("spaces", Spaces.screenSpacesMap)
         Logger.info("current space", Spaces.currentSpaceIndex, Spaces.currentSpaceId)

src/logic/events/SpacesEvents.swift

@@ -8,7 +8,7 @@ class SpacesEvents {
     @objc private static func handleEvent(_ notification: Notification) {
         Logger.debug(notification.name.rawValue)
         // if UI was kept open during Space transition, the Spaces may be obsolete; we refresh them
-        App.app.refreshOpenUi(Windows.list)
+        App.app.refreshOpenUi(Windows.list, .refreshUiAfterExternalEvent)
         Logger.info("current space", Spaces.currentSpaceIndex, Spaces.currentSpaceId)
         // from macos 12.2 beta onwards, we can't get other-space windows; grabbing windows when switching spaces mitigates the issue
         // also, updating windows on Space transition works around an issue with Safari where its fullscreen windows spawn not in fullscreen.

src/ui/App.swift

@@ -197,15 +197,15 @@ class App: AppCenterApplication {
         CGWarpMouseCursorPosition(point)
     }
 
-    func refreshOpenUi(_ windowsToScreenshot: [Window], skipUpdatesBeforeShowing: Bool = false, onlyUpdateScreenshots: Bool = false) {
+    func refreshOpenUi(_ windowsToScreenshot: [Window], _ source: RefreshCausedBy) {
         if !windowsToScreenshot.isEmpty && SystemPermissions.screenRecordingPermission == .granted
                && !Preferences.onlyShowApplications()
                && (!Appearance.hideThumbnails || Preferences.previewFocusedWindow) {
-            Windows.refreshThumbnails(windowsToScreenshot, onlyUpdateScreenshots)
-            return
+            Windows.refreshThumbnails(windowsToScreenshot, source)
+            if source == .refreshOnlyThumbnailsAfterShowUi { return }
         }
         guard appIsBeingUsed else { return }
-        if !skipUpdatesBeforeShowing {
+        if source == .refreshUiAfterExternalEvent {
             if !Windows.updatesBeforeShowing() { hideUi(); return }
         }
         guard appIsBeingUsed else { return }
@@ -262,10 +262,10 @@ class App: AppCenterApplication {
         thumbnailsPanel.makeKeyAndOrderFront(nil) // workaround: without this, switching between 2 screens make thumbnailPanel invisible
         KeyRepeatTimer.toggleRepeatingKeyNextWindow()
         guard appIsBeingUsed else { return }
-        refreshOpenUi([], skipUpdatesBeforeShowing: true) // first display with the data we have
+        refreshOpenUi([], .showUi)
         guard appIsBeingUsed else { return }
         thumbnailsPanel.show()
-        refreshOpenUi(Windows.list, skipUpdatesBeforeShowing: true, onlyUpdateScreenshots: true) // second refresh with fresh thumbnails
+        refreshOpenUi(Windows.list, .refreshOnlyThumbnailsAfterShowUi)
     }
 
     func checkIfShortcutsShouldBeDisabled(_ activeWindow: Window?, _ activeApp: NSRunningApplication?) {
@@ -338,3 +338,10 @@ extension App: NSApplicationDelegate {
         setNativeCommandTabEnabled(true)
     }
 }
+
+enum RefreshCausedBy {
+    case showUi
+    case refreshOnlyThumbnailsAfterShowUi
+    case refreshUiAfterThumbnailsHaveBeenRefreshed
+    case refreshUiAfterExternalEvent
+}

src/ui/main-window/PreviewPanel.swift

@@ -27,6 +27,13 @@ class PreviewPanel: NSPanel {
         setAccessibilitySubrole(.unknown)
     }
 
+    func updateImageIfShowing(_ id: CGWindowID?,  _ preview: NSImage, _ size: CGSize) {
+        if id == currentId && previewView.image != preview {
+            previewView.image = preview.copyToSeparateContexts()
+            previewView.image!.size = size
+        }
+    }
+
     func show(_ id: CGWindowID, _ preview: NSImage, _ position: CGPoint, _ size: CGSize) {
         if previewView.image != preview {
             previewView.image = preview.copyToSeparateContexts()