Use notifications to deal with NSScrollPocket

macos/Sources/Ghostty/SurfaceScrollView.swift

@@ -102,6 +102,19 @@ class SurfaceScrollView: NSView {
             self?.handleLiveScroll()
         })
         
+        // Listen for frame change events. See the docstring for
+        // handleFrameChange for why this is necessary.
+        observers.append(NotificationCenter.default.addObserver(
+            forName: NSView.frameDidChangeNotification,
+            object: nil,
+            // Since this observer is used to immediately override the event
+            // that produced the notification, we let it run synchronously on
+            // the posting thread.
+            queue: nil
+        ) { [weak self] notification in
+            self?.handleFrameChange(notification)
+        })
+
         // Listen for derived config changes to update scrollbar settings live
         surfaceView.$derivedConfig
             .sink { [weak self] _ in
@@ -134,20 +147,6 @@ class SurfaceScrollView: NSView {
     
     override func layout() {
         super.layout()
-
-        // The SwiftUI ScrollView host likes to add its own styling overlays to
-        // the titlebar area, which are incompatible with the hidden titlebar
-        // style. They won't be present when the app is first opened, but will
-        // appear when creating splits or cycling fullscreen. There's no public
-        // way to disable them in AppKit, so we just have to play whack-a-mole.
-        // See https://developer.apple.com/forums/thread/798392.
-        if window is HiddenTitlebarTerminalWindow {
-            for view in scrollView.subviews {
-                if view.className.contains("NSScrollPocket") {
-                    view.removeFromSuperview()
-                }
-            }
-        }
         
         // Fill entire bounds with scroll view
         scrollView.frame = bounds
@@ -283,6 +282,49 @@ class SurfaceScrollView: NSView {
         scrollView.reflectScrolledClipView(scrollView.contentView)
     }
 
+    /// Handles a change in the frame of NSScrollPocket styling overlays
+    ///
+    /// NSScrollView instances are set up with a subview hierarchy which, as far
+    /// as I can tell, is intended to add a blur effect to any part of a scroll
+    /// view that lies under the titlebar, presumably to complement a titlebar
+    /// using liquid glass transparency. This doesn't work correctly with our
+    /// hidden titlebar style, which does have a titlebar container, albeit
+    /// hidden. The styling overlays don't care and size themselves to this
+    /// container, creating a blurry, transparent field that clips the top of
+    /// the surface view.
+    ///
+    /// With other titlebar styles, these views always have zero frame size,
+    /// presumably because there is no overlap between the scroll view and the
+    /// titlebar container.
+    ///
+    /// In native fullscreen, the titlebar detaches from the window and these
+    /// views seem to work a bit differently, taking non-zero sizes for all
+    /// styles without creating any problems.
+    ///
+    /// To handle this in a way that minimizes the difference between how the
+    /// hidden titlebar and other window styles behave, we do as follows: If we
+    /// have the hidden titlebar style and we're not fullscreen, we listen to
+    /// frame changes on NSScrollPocket-related objects in scrollView.subviews,
+    /// and reset their frame to zero.
+    ///
+    /// See also https://developer.apple.com/forums/thread/798392.
+    private func handleFrameChange(_ notification: Notification) {
+        guard let window = window as? HiddenTitlebarTerminalWindow else { return }
+        guard !window.styleMask.contains(.fullScreen) else { return }
+        guard let view = notification.object as? NSView else { return }
+        guard view.className.contains("NSScrollPocket") else { return }
+        guard scrollView.subviews.contains(view) else { return }
+        // These guards to avoid an infinite loop don't actually seem necessary.
+        // The number of times we reach this point during any given event (e.g.,
+        // creating a split) is the same either way. We keep them anyway out of
+        // an abundance of caution.
+        view.postsFrameChangedNotifications = false
+        view.frame = NSRect(x: 0, y: 0, width: 0, height: 0)
+        view.postsFrameChangedNotifications = true
+    }
+
+    // MARK: Calculations
+
     /// Calculate the appropriate document view height given a scrollbar state
     private func documentHeight(_ scrollbar: Ghostty.Action.Scrollbar?) -> CGFloat {
         let contentHeight = scrollView.contentSize.height