Fix macOS PiP window not resizing when switching videos

Switching videos during active PiP left the window at the old video's
size with white space around the new content, for two reasons:

- MPV reports the new video's width and height as separate property
  events while the old values are kept across switches, so a transient
  mixed size (new width with old height) could reach the PiP capture
  path and AVKit sized the window from that frame. Coalesce the PiP
  capture-size/aspect update with a short debounce so only settled
  dimensions are used.

- On macOS nothing told AVKit about the new aspect: unlike iOS, the
  bridge never flushed the sample buffer renderer on an aspect change
  during active PiP, and AVKit doesn't resize the macOS PiP window on
  its own. Flush the renderer, reset the format description, and
  resize the PiP window to the new aspect ratio explicitly.

Yattee/Services/Player/MPV/MPVPiPBridge.swift

@@ -222,21 +222,40 @@ final class MPVPiPBridge: NSObject {
     /// - Parameter aspectRatio: Video width divided by height (e.g., 16/9 = 1.777...)
     func updateVideoAspectRatio(_ aspectRatio: CGFloat) {
         guard aspectRatio > 0 else { return }
+        #if os(macOS)
+        let previousAspectRatio = videoAspectRatio
+        #endif
         videoAspectRatio = aspectRatio
 
         #if os(macOS)
-        // On macOS, update layer bounds to match aspect ratio
-        // This helps AVKit size the PiP window correctly
-        let currentBounds = sampleBufferLayer.bounds
-        let newHeight = currentBounds.width / aspectRatio
-        let newBounds = CGRect(x: 0, y: 0, width: currentBounds.width, height: newHeight)
+        if isPiPActive {
+            // Video changed while PiP is active. AVKit sizes the PiP window from
+            // the format of the initially enqueued frames and doesn't resize it
+            // when frames with new dimensions arrive, leaving empty space around
+            // the new video. Flush stale frames, force a fresh format description
+            // so AVKit re-reads the new dimensions, and resize the PiP window to
+            // the new aspect ratio ourselves.
+            let ratioChange = abs(aspectRatio - previousAspectRatio) / previousAspectRatio
+            if ratioChange > 0.01 {
+                sampleBufferLayer.sampleBufferRenderer.flush()
+                currentFormatDescription = nil
+                resizePiPWindow(toAspectRatio: aspectRatio)
+                LoggingService.shared.debug("MPVPiPBridge: Aspect ratio changed during PiP \(previousAspectRatio) -> \(aspectRatio), flushed buffer and resized PiP window", category: .mpv)
+            }
+        } else {
+            // On macOS, update layer bounds to match aspect ratio
+            // This helps AVKit size the PiP window correctly
+            let currentBounds = sampleBufferLayer.bounds
+            let newHeight = currentBounds.width / aspectRatio
+            let newBounds = CGRect(x: 0, y: 0, width: currentBounds.width, height: newHeight)
 
-        CATransaction.begin()
-        CATransaction.setDisableActions(true)
-        sampleBufferLayer.bounds = newBounds
-        CATransaction.commit()
+            CATransaction.begin()
+            CATransaction.setDisableActions(true)
+            sampleBufferLayer.bounds = newBounds
+            CATransaction.commit()
 
-        LoggingService.shared.debug("MPVPiPBridge: Updated aspect ratio to \(aspectRatio), layer bounds: \(newBounds)", category: .mpv)
+            LoggingService.shared.debug("MPVPiPBridge: Updated aspect ratio to \(aspectRatio), layer bounds: \(newBounds)", category: .mpv)
+        }
         #else
         // On iOS, don't modify bounds when PiP is inactive - this causes frame misalignment
         // (negative Y offset) which breaks the system's PiP restore UI positioning.
@@ -857,6 +876,29 @@ extension MPVPiPBridge {
         return nil
     }
 
+    /// Resize the PiP window to match a new video aspect ratio, keeping the
+    /// current width and top edge. AVKit doesn't resize the window itself when
+    /// the enqueued frame dimensions change (e.g. switching videos during PiP).
+    func resizePiPWindow(toAspectRatio aspectRatio: CGFloat) {
+        guard aspectRatio > 0, let pipWindow = findPiPWindow() else {
+            LoggingService.shared.debug("MPVPiPBridge: resizePiPWindow - PiP window not found", category: .mpv)
+            return
+        }
+
+        let contentRect = pipWindow.contentRect(forFrameRect: pipWindow.frame)
+        guard contentRect.width > 0 else { return }
+        let newContentHeight = contentRect.width / aspectRatio
+        let heightDelta = newContentHeight - contentRect.height
+        guard abs(heightDelta) >= 1 else { return }
+
+        var frame = pipWindow.frame
+        frame.size.height += heightDelta
+        frame.origin.y -= heightDelta // keep the top edge in place
+        pipWindow.setFrame(frame, display: true, animate: false)
+
+        LoggingService.shared.debug("MPVPiPBridge: Resized PiP window for aspect \(aspectRatio): \(frame)", category: .mpv)
+    }
+
     /// Recursively log view hierarchy for debugging
     private func logViewHierarchy(_ view: NSView, depth: Int) {
         let indent = String(repeating: "  ", count: depth)