From e2258a26df332658da7f5abfa64b206f39fddf5d Mon Sep 17 00:00:00 2001
From: foxnne <foxnne@gmail.com>
Date: Sun, 18 Oct 2026 14:10:57 -0500
Subject: [PATCH] macos: a window's glass bevel follows a live resize

The frost and colour of a window of Liquid Glass are masked by the clearing
bevel, a mask layer sized by each frame's look. A live resize grows the parts
at once, while the mask stayed at its old size until the next frame. The mask
is anchored at the bottom, so as the window grew, the frost along its top was
masked out for a frame and the old mask's rounded corner showed as a
diagonal, appearing and disappearing as the window was dragged. Each masked
part now sizes its mask as AppKit resizes it, through one frame-changed
observer, and unregisters when the glass is taken away.
---
 .../src/platform/macos/visual_effect_view.m   | 36 ++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/backend/src/platform/macos/visual_effect_view.m b/backend/src/platform/macos/visual_effect_view.m
index 0cfa5cfb3..07e56d377 100644
--- a/backend/src/platform/macos/visual_effect_view.m
+++ b/backend/src/platform/macos/visual_effect_view.m
@@ -1261,6 +1261,34 @@ int fizzy_macos_window_has_liquid_glass(void *nswindow) {
  */
 /* The glass parts beside SDL's view in `window`'s frame view (`window_glass_ids`), once; 1 where they
  * are (or already were), 0 where the OS has no Liquid Glass. A titled window's and a menu's alike. */
+/* A window glass part's feather mask (`windowFeatherMask`) sized with the part as AppKit resizes
+ * it, not at the next frame's look: a mask left at the old size for a frame, anchored at the
+ * bottom, masked the frost out along the top of a window growing under a live resize, its old
+ * corner showing as a diagonal (the user). One observer for every part
+ * (`windowGlassMaskFollows`), each part's registration gone with it (`windowGlassRemove`). */
+@interface FizzyGlassMaskFollower : NSObject
+@end
+
+@implementation FizzyGlassMaskFollower
+- (void)partResized:(NSNotification *)note {
+    CALayer *layer = [(NSView *)[note object] layer];
+    CALayer *mask = [layer mask];
+    if (mask == nil || CGRectEqualToRect([mask frame], [layer bounds])) return;
+    [CATransaction begin];
+    [CATransaction setDisableActions:YES];
+    [mask setFrame:[layer bounds]];
+    [CATransaction commit];
+}
+@end
+
+static FizzyGlassMaskFollower *g_glass_mask_follower = nil;
+
+static void windowGlassMaskFollows(NSView *part) {
+    if (g_glass_mask_follower == nil) g_glass_mask_follower = [[FizzyGlassMaskFollower alloc] init];
+    [part setPostsFrameChangedNotifications:YES];
+    [[NSNotificationCenter defaultCenter] addObserver:g_glass_mask_follower selector:@selector(partResized:) name:NSViewFrameDidChangeNotification object:part];
+}
+
 static int windowGlassInstall(NSWindow *window, long blur_material) {
     if (window == nil || !fizzy_macos_liquid_glass_available()) return 0;
     if (windowGlassPart(window, window_glass_under) != nil) return 1;
@@ -1301,6 +1329,7 @@ static int windowGlassInstall(NSWindow *window, long blur_material) {
         /* Its own layer, which keeps the feather's mask: one AppKit lends a view not asking
          * for a layer (the glass, the blur) drops the mask set on it. */
         [part setWantsLayer:YES];
+        if (k == window_glass_over || k == window_glass_fill) windowGlassMaskFollows(part);
         [frame addSubview:part positioned:NSWindowBelow relativeTo:content];
     }
     return 1;
@@ -1309,7 +1338,12 @@ static int windowGlassInstall(NSWindow *window, long blur_material) {
 /* `window`'s glass parts gone (`windowGlassInstall`): a plain dvui app's window made of another
  * material (`fizzy_macos_window_plain_material`). */
 static void windowGlassRemove(NSWindow *window) {
-    for (int k = 0; k < window_glass_parts; k++) [windowGlassPart(window, k) removeFromSuperview];
+    for (int k = 0; k < window_glass_parts; k++) {
+        NSView *part = windowGlassPart(window, k);
+        if (part == nil) continue;
+        if (g_glass_mask_follower != nil) [[NSNotificationCenter defaultCenter] removeObserver:g_glass_mask_follower name:NSViewFrameDidChangeNotification object:part];
+        [part removeFromSuperview];
+    }
 }
 
 int fizzy_macos_window_liquid_glass(void *nswindow, long blur_material) {