Streamline drawn decorations management to avoid call ordering problems (#20840)

src/Avalonia.Controls/TopLevelHost.Decorations.cs

@@ -48,23 +48,33 @@ internal partial class TopLevelHost
     internal WindowDrawnDecorations? Decorations => _decorations;
 
     /// <summary>
-    /// Enables drawn window decorations with the specified parts.
-    /// Creates the decorations instance, applies the template, and inserts layers into the visual tree.
+    /// Updates drawn window decorations with the specified parts and window state.
+    /// When <paramref name="parts"/> is <c>null</c>, decorations are removed entirely.
+    /// When non-null (including <see cref="DrawnWindowDecorationParts.None"/>), the decoration
+    /// infrastructure is kept alive and parts/fullscreen state are updated.
     /// </summary>
-    internal void EnableDecorations(DrawnWindowDecorationParts parts)
+    internal void UpdateDrawnDecorations(DrawnWindowDecorationParts? parts, WindowState windowState)
     {
+        if (parts == null)
+        {
+            RemoveDecorations();
+            return;
+        }
+
+        var enabledParts = parts.Value;
+
         if (_decorations != null)
         {
             // Layers persist across part changes; pseudo-classes driven by EnabledParts
             // control visibility of individual decoration elements in the theme.
-            _decorations.EnabledParts = parts;
+            _decorations.EnabledParts = enabledParts;
             if (_resizeGrips != null)
-                _resizeGrips.IsVisible = parts.HasFlag(DrawnWindowDecorationParts.ResizeGrips);
-            return;
+                _resizeGrips.IsVisible = enabledParts.HasFlag(DrawnWindowDecorationParts.ResizeGrips);
         }
-
+        else
+        {
             _decorations = new WindowDrawnDecorations();
-        _decorations.EnabledParts = parts;
+            _decorations.EnabledParts = enabledParts;
 
             // Set up logical parenting
             LogicalChildren.Add(_decorations);
@@ -85,7 +95,7 @@ internal partial class TopLevelHost
 
             // Always create resize grips; visibility is controlled by EnabledParts
             _resizeGrips = new ResizeGripLayer();
-        _resizeGrips.IsVisible = parts.HasFlag(DrawnWindowDecorationParts.ResizeGrips);
+            _resizeGrips.IsVisible = enabledParts.HasFlag(DrawnWindowDecorationParts.ResizeGrips);
             VisualChildren.Add(_resizeGrips);
 
             // Attach to window if available
@@ -99,13 +109,15 @@ internal partial class TopLevelHost
 
             ApplyDecorationsTemplate();
             InvalidateMeasure();
-        _decorationsOverlayPeer?.InvalidateChildren();
+        }
+
+        ApplyFullscreenState(windowState == WindowState.FullScreen);
     }
 
     /// <summary>
-    /// Disables drawn window decorations and removes all layers.
+    /// Removes drawn window decorations and all associated layers.
     /// </summary>
-    internal void DisableDecorations()
+    private void RemoveDecorations()
     {
         if (_decorations == null)
             return;
@@ -192,15 +204,15 @@ internal partial class TopLevelHost
     }
 
     /// <summary>
-    /// Shows or hides the fullscreen popover based on the window state.
-    /// Called by Window when window state changes.
+    /// Applies fullscreen-specific layer visibility: hides overlay/underlay and enables
+    /// popover hover detection, or restores normal state.
     /// </summary>
-    internal void SetFullscreenPopoverEnabled(bool enabled)
+    private void ApplyFullscreenState(bool isFullscreen)
     {
         if (_fullscreenPopover == null)
             return;
 
-        if (enabled)
+        if (isFullscreen)
         {
             // In fullscreen mode, hide overlay and underlay, enable popover hover detection
             if (_overlay != null)

src/Avalonia.Controls/Window.cs

@@ -626,10 +626,7 @@ namespace Avalonia.Controls
                 StartRendering();
             }
 
-            // Update fullscreen popover visibility
-            TopLevelHost.SetFullscreenPopoverEnabled(state == WindowState.FullScreen);
-
-            // Update decoration parts for the new window state
+            // Update decoration parts and fullscreen popover state for the new window state
             UpdateDrawnDecorationParts();
         }
 
@@ -643,13 +640,11 @@ namespace Avalonia.Controls
         
         private void UpdateDrawnDecorations()
         {
-            var needsDrawnDecorations = PlatformImpl?.NeedsManagedDecorations ?? false;
+            var parts = ComputeDecorationParts();
+            TopLevelHost.UpdateDrawnDecorations(parts, WindowState);
 
-            var parts = needsDrawnDecorations ? ComputeDecorationParts() : DrawnWindowDecorationParts.None;
-            if (parts != DrawnWindowDecorationParts.None)
+            if (parts != null)
             {
-                TopLevelHost.EnableDecorations(parts);
-
                 // Forward ExtendClientAreaTitleBarHeightHint to decoration TitleBarHeight
                 var decorations = TopLevelHost.Decorations;
                 if (decorations != null)
@@ -659,10 +654,6 @@ namespace Avalonia.Controls
                         decorations.TitleBarHeightOverride = hint;
                 }
             }
-            else
-            {
-                TopLevelHost.DisableDecorations();
-            }
             
             UpdateDrawnDecorationMargins();
         }
@@ -676,11 +667,14 @@ namespace Avalonia.Controls
             if (TopLevelHost.Decorations == null)
                 return;
 
-            TopLevelHost.EnableDecorations(ComputeDecorationParts());
+            TopLevelHost.UpdateDrawnDecorations(ComputeDecorationParts(), WindowState);
         }
 
-        private Chrome.DrawnWindowDecorationParts ComputeDecorationParts()
+        private Chrome.DrawnWindowDecorationParts? ComputeDecorationParts()
         {
+            if (!(PlatformImpl?.NeedsManagedDecorations ?? false))
+                return null;
+
             var platformNeeds = PlatformImpl?.RequestedDrawnDecorations ?? PlatformRequestedDrawnDecoration.None;
             var parts = Chrome.DrawnWindowDecorationParts.None;
             if (WindowDecorations != WindowDecorations.None)