Fixes #4843. Button: avoid create-then-destroy shadow allocation via CWP InitializingShadowStyle event

Terminal.Gui/ViewBase/Adornment/ArrangerButton.cs

@@ -74,7 +74,6 @@ public ArrangerButton ()
         Height = 1;
         NoDecorations = true;
         NoPadding = true;
-        base.ShadowStyle = null;
         base.Visible = false;
 
         AddCommand (Command.Up, DefaultAcceptHandler);
@@ -85,6 +84,10 @@ public ArrangerButton ()
         _orientationHelper = new OrientationHelper (this);
     }
 
+    /// <inheritdoc/>
+    /// <remarks>Returns <see langword="null"/> so that no shadow infrastructure is ever allocated for arranger buttons.</remarks>
+    protected override ShadowStyles? GetDefaultShadowStyle () => null;
+
     private ArrangeButtons _buttonType = (ArrangeButtons)(-1);
 
     /// <summary>

Terminal.Gui/Views/Button.cs

@@ -84,10 +84,25 @@ public Button ()
 
         TitleChanged += Button_TitleChanged;
 
-        base.ShadowStyle = DefaultShadow;
+        // Apply the initial shadow via a virtual method so that subclasses (e.g. ScrollButton)
+        // can override GetDefaultShadowStyle() to return null, completely avoiding the
+        // create-then-destroy allocation cycle that would otherwise occur.
+        base.ShadowStyle = GetDefaultShadowStyle ();
         MouseHighlightStates = DefaultMouseHighlightStates;
     }
 
+    /// <summary>
+    ///     Returns the shadow style that should be applied during construction. Subclasses that
+    ///     never show a shadow (e.g. <see cref="ScrollButton"/>, <see cref="ArrangerButton"/>) should
+    ///     override this to return <see langword="null"/> to avoid the create-then-destroy allocation
+    ///     pattern.
+    /// </summary>
+    /// <returns>
+    ///     <see cref="DefaultShadow"/> by default. Return <see langword="null"/> to construct without
+    ///     any shadow infrastructure.
+    /// </returns>
+    protected virtual ShadowStyles? GetDefaultShadowStyle () => DefaultShadow;
+
     /// <inheritdoc/>
     protected override void OnMouseHoldRepeatChanged (ValueChangedEventArgs<MouseFlags?> args) => SetMouseBindings (args.NewValue);
 

Terminal.Gui/Views/ScrollBar/ScrollButton.cs

@@ -60,14 +60,17 @@ public ScrollButton ()
         CanFocus = false;
         NoDecorations = true;
         NoPadding = true;
-        base.ShadowStyle = null;
         MouseHoldRepeat = MouseFlags.LeftButtonReleased;
 
         // ReSharper disable once UseObjectOrCollectionInitializer
         _orientationHelper = new OrientationHelper (this);
         SetGlyph ();
     }
 
+    /// <inheritdoc/>
+    /// <remarks>Returns <see langword="null"/> so that no shadow infrastructure is ever allocated for scroll buttons.</remarks>
+    protected override ShadowStyles? GetDefaultShadowStyle () => null;
+
     /// <summary>
     ///     Gets or sets the direction this <see cref="ScrollButton"/> scrolls.
     /// </summary>

Tests/UnitTestsParallelizable/Views/ButtonTests.cs

@@ -1005,4 +1005,73 @@ public void Mouse_Click_Does_Not_Raise_Activating ()
 
         button.Dispose ();
     }
+
+    // Copilot
+    /// <summary>
+    ///     Verifies that a newly-constructed Button has shadow infrastructure allocated by default,
+    ///     because <see cref="Button.GetDefaultShadowStyle"/> returns <see cref="Button.DefaultShadow"/>
+    ///     and is called in the constructor.
+    /// </summary>
+    [Fact]
+    public void ShadowStyle_Applied_In_Constructor_By_Default ()
+    {
+        Button button = new ();
+
+        // Default shadow is applied in the constructor via GetDefaultShadowStyle().
+        Assert.Equal (Button.DefaultShadow, button.ShadowStyle);
+        Assert.NotNull (button.Margin.View); // MarginView was created.
+
+        button.Dispose ();
+    }
+
+    // Copilot
+    /// <summary>
+    ///     Verifies that setting <see cref="View.ShadowStyle"/> to <see langword="null"/> after construction
+    ///     removes the shadow infrastructure.
+    /// </summary>
+    [Fact]
+    public void ShadowStyle_Null_After_Construction_Removes_Shadow ()
+    {
+        Button button = new ();
+        button.ShadowStyle = null;
+
+        Assert.Null (button.ShadowStyle);
+
+        button.Dispose ();
+    }
+
+    // Copilot
+    /// <summary>
+    ///     Verifies that <see cref="ScrollButton"/> does not create shadow infrastructure during construction
+    ///     because <see cref="ScrollButton.GetDefaultShadowStyle"/> returns <see langword="null"/>,
+    ///     eliminating the create-then-destroy allocation pattern described in the issue.
+    /// </summary>
+    [Fact]
+    public void ScrollButton_GetDefaultShadowStyle_Returns_Null_No_MarginView ()
+    {
+        ScrollButton btn = new ();
+
+        // GetDefaultShadowStyle() returns null → Button constructor sets ShadowStyle = null
+        // → no shadow infrastructure is ever created.
+        Assert.Null (btn.ShadowStyle);
+        Assert.Null (btn.Margin.View);
+
+        btn.Dispose ();
+    }
+
+    // Copilot
+    /// <summary>
+    ///     Verifies that explicitly setting <see cref="View.ShadowStyle"/> to a non-default value
+    ///     after construction correctly replaces the initial default.
+    /// </summary>
+    [Fact]
+    public void ShadowStyle_ExplicitValue_After_Construction_IsApplied ()
+    {
+        Button button = new ();
+        button.ShadowStyle = ShadowStyles.Transparent;
+
+        Assert.Equal (ShadowStyles.Transparent, button.ShadowStyle);
+
+        button.Dispose ();
+    }
 }