fix: collapse shadow padding when window touches a screen edge (#2717)

The custom window frame draws its shadow inside a 12px transparent
padding that belongs to the window bounds, so window managers clamp
the padded bounds against the screen edge and the visible content
stops short of it. Collapse the padding on the touching side to let
the content sit flush against the edge.

Author: Gadfly

src/Views/ChromelessWindow.cs

@@ -26,6 +26,8 @@ public ChromelessWindow()
         {
             Focusable = true;
             Native.OS.SetupForWindow(this);
+            PositionChanged += (_, _) => UpdateEdgeSnapInsets();
+            Resized += (_, _) => UpdateEdgeSnapInsets();
         }
 
         public void BeginMoveWindow(object _, PointerPressedEventArgs e)
@@ -89,6 +91,9 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
 
             if (OperatingSystem.IsWindows() && change.Property == WindowStateProperty)
                 Native.Win64Utilities.FixWindowFrame(this);
+
+            if (change.Property == WindowStateProperty)
+                UpdateEdgeSnapInsets();
         }
 
         protected override void OnKeyDown(KeyEventArgs e)
@@ -132,5 +137,47 @@ private void OnWindowBorderPointerPressed(object sender, PointerPressedEventArgs
             if (sender is Border { Tag: WindowEdge edge } && CanResize)
                 BeginResizeDrag(edge, e);
         }
+
+        // The transparent padding around the window holds the self-drawn shadow
+        // (keep the value in sync with "Window.custom_window_frame" style).
+        // Collapse it on the side touching a screen edge so the content sits flush.
+        private void UpdateEdgeSnapInsets()
+        {
+            if (!Classes.Contains("custom_window_frame"))
+                return;
+
+            if (WindowState != WindowState.Normal)
+            {
+                // Let the "WindowState=Maximized" style take over
+                ClearValue(PaddingProperty);
+                return;
+            }
+
+            var screen = Screens.ScreenFromWindow(this);
+            if (screen == null)
+                return;
+
+            var workArea = screen.WorkingArea;
+            var right = Position.X + Bounds.Width;
+            var bottom = Position.Y + Bounds.Height;
+
+            const double tolerance = 2.0;
+            var snapTop = Position.Y <= workArea.Y + tolerance;
+            var snapLeft = Position.X <= workArea.X + tolerance;
+            var snapRight = right >= workArea.Right - tolerance;
+            var snapBottom = bottom >= workArea.Bottom - tolerance;
+
+            if (!snapTop && !snapLeft && !snapRight && !snapBottom)
+            {
+                ClearValue(PaddingProperty);
+                return;
+            }
+
+            Padding = new Thickness(
+                snapLeft ? 0 : 12,
+                snapTop ? 0 : 12,
+                snapRight ? 0 : 12,
+                snapBottom ? 0 : 12);
+        }
     }
 }