feat: smoother rounded corners for the main window

src/ui/App.swift

@@ -97,7 +97,7 @@ class App: AppCenterApplication, NSApplicationDelegate {
     // we put application code here which should be executed on init() and Preferences change
     func resetPreferencesDependentComponents() {
         ThumbnailsView.recycledViews = ThumbnailsView.recycledViews.map { _ in ThumbnailView() }
-        thumbnailsPanel.thumbnailsView.layer!.cornerRadius = Preferences.windowCornerRadius
+        thumbnailsPanel.thumbnailsView.updateRoundedCorners(Preferences.windowCornerRadius)
     }
 
     func restart() {

src/ui/main-window/ThumbnailsPanel.swift

@@ -13,7 +13,7 @@ class ThumbnailsPanel: NSPanel, NSWindowDelegate {
         hasShadow = false
         titleVisibility = .hidden
         backgroundColor = .clear
-        contentView!.addSubview(thumbnailsView)
+        contentView! = thumbnailsView
         // triggering AltTab before or during Space transition animation brings the window on the Space post-transition
         collectionBehavior = .canJoinAllSpaces
         // 2nd highest level possible; this allows the app to go on top of context menus

src/ui/main-window/ThumbnailsView.swift

@@ -10,12 +10,26 @@ class ThumbnailsView: NSVisualEffectView {
         material = Preferences.windowMaterial
         state = .active
         wantsLayer = true
-        layer!.cornerRadius = Preferences.windowCornerRadius
+        updateRoundedCorners(Preferences.windowCornerRadius)
         addSubview(scrollView)
         // TODO: think about this optimization more
         (1...100).forEach { _ in ThumbnailsView.recycledViews.append(ThumbnailView()) }
     }
 
+    /// using layer!.cornerRadius works but the corners are aliased; this custom approach gives smooth rounded corners
+    /// see https://stackoverflow.com/a/29386935/2249756
+    func updateRoundedCorners(_ cornerRadius: CGFloat) {
+        let edgeLength = 2.0 * cornerRadius + 1.0
+        maskImage = NSImage(size: NSSize(width: edgeLength, height: edgeLength), flipped: false) { rect in
+            let bezierPath = NSBezierPath(roundedRect: rect, xRadius: cornerRadius, yRadius: cornerRadius)
+            NSColor.black.set()
+            bezierPath.fill()
+            return true
+        }
+        maskImage!.capInsets = NSEdgeInsets(top: cornerRadius, left: cornerRadius, bottom: cornerRadius, right: cornerRadius)
+        maskImage!.resizingMode = .stretch
+    }
+
     func nextRow(_ direction: Direction) -> [ThumbnailView]? {
         let step = direction == .down ? 1 : -1
         if let currentRow = Windows.focusedWindow()?.row {