feat: add fade-in for window previews (closes #2456)

src/logic/Windows.swift

@@ -152,6 +152,7 @@ class Windows {
             !Preferences.onlyShowApplications(),
             App.app.appIsBeingUsed && App.app.thumbnailsPanel.isKeyWindow,
             let window = focusedWindow(),
+            let id = window.cgWindowId,
             let preview = window.getPreview(),
             let position = window.position,
             let size = window.size
@@ -159,11 +160,7 @@ class Windows {
             App.app.previewPanel.orderOut(nil)
             return
         }
-        App.app.previewPanel.setPreview(preview)
-        var frame = NSRect(origin: position, size: size)
-        frame.origin.y = NSScreen.preferred().frame.maxY - frame.maxY
-        App.app.previewPanel.setFrame(frame, display: false)
-        App.app.previewPanel.order(.below, relativeTo: App.app.thumbnailsPanel.windowNumber)
+        App.app.previewPanel.show(id, preview, position, size)
     }
 
     static func voiceOverWindow(_ windowIndex: Int = focusedWindowIndex) {

src/ui/main-window/PreviewPanel.swift

@@ -3,6 +3,7 @@ import Cocoa
 class PreviewPanel: NSPanel {
     private let previewView = NSImageView()
     private let borderView = BorderView()
+    private var currentId: CGWindowID?
 
     convenience init() {
         self.init(contentRect: .zero, styleMask: [.nonactivatingPanel, .titled, .fullSizeContentView], backing: .buffered, defer: false)
@@ -24,8 +25,22 @@ class PreviewPanel: NSPanel {
         setAccessibilitySubrole(.unknown)
     }
 
-    func setPreview(_ preview: NSImage) {
-        previewView.image = preview
+    func show(_ id: CGWindowID, _ preview: NSImage, _ position: CGPoint, _ size: CGSize) {
+        if id != currentId  {
+            previewView.image = preview
+            var frame = NSRect(origin: position, size: size)
+            frame.origin.y = NSScreen.preferred().frame.maxY - frame.maxY
+            setFrame(frame, display: false)
+        }
+        if id != currentId || !isVisible {
+            alphaValue = 0
+            NSAnimationContext.runAnimationGroup { context in
+                context.duration = 0.3
+                animator().alphaValue = 1
+            }
+            currentId = id
+            order(.below, relativeTo: App.app.thumbnailsPanel.windowNumber)
+        }
     }
 }