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 +84,13 @@ public ArrangerButton ()
         _orientationHelper = new OrientationHelper (this);
     }
 
+    /// <inheritdoc/>
+    /// <remarks>
+    ///     Sets <see cref="ValueChangingEventArgs{T}.NewValue"/> to <see langword="null"/> so that no shadow infrastructure is
+    ///     allocated by default for arranger buttons.
+    /// </remarks>
+    protected override void OnInitializingShadowStyle (ValueChangingEventArgs<ShadowStyles?> args) => args.NewValue = null;
+
     private ArrangeButtons _buttonType = (ArrangeButtons)(-1);
 
     /// <summary>

Terminal.Gui/ViewBase/Adornment/ShadowView.cs

@@ -184,7 +184,7 @@ private Attribute GetAttributeUnderLocation (Point location)
         Attribute attr = attribute.Value;
 
         var newAttribute = new Attribute (ShadowStyle == ShadowStyles.Opaque ? Color.Black : attr.Foreground.GetDimmerColor (),
-                                          ShadowStyle == ShadowStyles.Opaque ? attr.Background : attr.Background.GetDimmerColor (0.05),
+                                          ShadowStyle == ShadowStyles.Opaque ? attr.Background : attr.Background.GetDimmerColor (0.9),
                                           attr.Style);
 
         // If the BG is DarkGray, GetDimmerColor gave up. Instead of using the attribute in the Driver under the shadow,
@@ -198,7 +198,7 @@ private Attribute GetAttributeUnderLocation (Point location)
         attr = underView?.GetAttributeForRole (VisualRole.Normal) ?? Attribute.Default;
 
         newAttribute = new Attribute (ShadowStyle == ShadowStyles.Opaque ? Color.Black : attr.Background.GetDimmerColor (),
-                                      ShadowStyle == ShadowStyles.Opaque ? attr.Background : attr.Foreground.GetDimmerColor (0.25),
+                                      ShadowStyle == ShadowStyles.Opaque ? attr.Background : attr.Foreground.GetDimmerColor (0.9),
                                       attr.Style);
 
         return newAttribute;

Terminal.Gui/Views/Button.cs

@@ -1,8 +1,9 @@
 namespace Terminal.Gui.Views;
 
 /// <summary>
-///     Raises the <see cref="View.Accepting"/> and <see cref="View.Accepted"/> events when the user presses <see cref="View.HotKey"/>,
-///         <c>Enter</c>, or <c>Space</c> or clicks with the mouse.
+///     Raises the <see cref="View.Accepting"/> and <see cref="View.Accepted"/> events when the user presses
+///     <see cref="View.HotKey"/>,
+///     <c>Enter</c>, or <c>Space</c> or clicks with the mouse.
 /// </summary>
 /// <remarks>
 ///     <para>Use <see cref="View.HotKeySpecifier"/> to change the hot key specifier from the default of ('_').</para>
@@ -84,10 +85,51 @@ public Button ()
 
         TitleChanged += Button_TitleChanged;
 
-        base.ShadowStyle = DefaultShadow;
+        // Determine and apply the initial shadow via the CWP InitializingShadowStyle event, so that
+        // subclasses and external subscribers can change or suppress the default allocation
+        // before any shadow infrastructure is created.
+        RaiseInitializingShadowStyle ();
         MouseHighlightStates = DefaultMouseHighlightStates;
     }
 
+    /// <summary>
+    ///     Called before the Button's initial <see cref="View.ShadowStyle"/> is applied during construction.
+    ///     Override to change or suppress the default shadow — set <see cref="ValueChangingEventArgs{T}.NewValue"/>
+    ///     to the desired style, or set <see cref="ValueChangingEventArgs{T}.Handled"/> to
+    ///     <see langword="true"/> to skip applying any shadow.
+    /// </summary>
+    /// <param name="args">
+    ///     Event args whose <see cref="ValueChangingEventArgs{T}.NewValue"/> is pre-set to
+    ///     <see cref="DefaultShadow"/>. Subclasses that never display a shadow
+    ///     (e.g. <see cref="ScrollButton"/> or internal buttons used by arrangement UI)
+    ///     should set <c>args.NewValue = null</c> to avoid the create-then-destroy allocation pattern.
+    /// </param>
+    protected virtual void OnInitializingShadowStyle (ValueChangingEventArgs<ShadowStyles?> args) { }
+
+    /// <summary>
+    ///     Fired before the Button's initial <see cref="View.ShadowStyle"/> is applied during construction.
+    ///     Subscribers can modify <see cref="ValueChangingEventArgs{T}.NewValue"/> or set
+    ///     <see cref="ValueChangingEventArgs{T}.Handled"/> to <see langword="true"/> to suppress the shadow.
+    /// </summary>
+    public event EventHandler<ValueChangingEventArgs<ShadowStyles?>>? InitializingShadowStyle;
+
+    private void RaiseInitializingShadowStyle ()
+    {
+        ValueChangingEventArgs<ShadowStyles?> args = new (null, DefaultShadow);
+
+        // 1. Virtual method — subclasses override to change/suppress the default shadow.
+        OnInitializingShadowStyle (args);
+
+        // 2. Event — external subscribers get a chance to customize.
+        InitializingShadowStyle?.Invoke (this, args);
+
+        // 3. Apply the (potentially modified) shadow style unless already handled.
+        if (!args.Handled)
+        {
+            base.ShadowStyle = args.NewValue;
+        }
+    }
+
     /// <inheritdoc/>
     protected override void OnMouseHoldRepeatChanged (ValueChangedEventArgs<MouseFlags?> args) => SetMouseBindings (args.NewValue);
 
@@ -118,11 +160,8 @@ private void SetMouseBindings (MouseFlags? mouseHoldRepeat)
         }
     }
 
-    /// <inheritdoc />
-    protected override void OnHotKeyCommand (ICommandContext? commandContext)
-    {
-        InvokeCommand (Command.Accept);
-    }
+    /// <inheritdoc/>
+    protected override void OnHotKeyCommand (ICommandContext? commandContext) => InvokeCommand (Command.Accept);
 
     private void Button_TitleChanged (object? sender, EventArgs<string> e)
     {
@@ -136,7 +175,7 @@ private void Button_TitleChanged (object? sender, EventArgs<string> e)
     /// <inheritdoc/>
     public override Rune HotKeySpecifier { get => base.HotKeySpecifier; set => TextFormatter.HotKeySpecifier = base.HotKeySpecifier = value; }
 
-    /// <inheritdoc />
+    /// <inheritdoc/>
     public bool IsDefault
     {
         get;

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>Sets <see cref="ValueChangingEventArgs{T}.NewValue"/> to <see langword="null"/> so that no shadow infrastructure is allocated by default for scroll buttons.</remarks>
+    protected override void OnInitializingShadowStyle (ValueChangingEventArgs<ShadowStyles?> args) => args.NewValue = null;
+
     /// <summary>
     ///     Gets or sets the direction this <see cref="ScrollButton"/> scrolls.
     /// </summary>

Tests/UnitTestsParallelizable/ViewBase/Adornment/ShadowTests.cs

@@ -572,7 +572,7 @@ public void TransparentShadow_OverWide_Draws_Transparent_At_Driver_Output ()
         output.WriteLine (output1);
 
         DriverAssert.AssertDriverOutputIs ("""
-                                           \x1b[30m\x1b[107m*\x1b[90m\x1b[107m \x1b[97m\x1b[40m \x1b[93m\x1b[100m \x1b[97m\x1b[40m🍎
+                                           \x1b[30m\x1b[107m*\x1b[90m\x1b[40m \x1b[97m\x1b[40m \x1b[93m\x1b[100m \x1b[97m\x1b[40m🍎
                                            """,
                                            output,
                                            app.Driver);

Tests/UnitTestsParallelizable/Views/ButtonTests.cs

@@ -1005,4 +1005,79 @@ 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.RaiseInitializingShadowStyle"/> fires with
+    ///     <see cref="Button.DefaultShadow"/> and no handler suppresses it.
+    /// </summary>
+    [Fact]
+    public void ShadowStyle_Applied_In_Constructor_By_Default ()
+    {
+        Button button = new ();
+
+        // Default shadow is applied in the constructor via the InitializingShadowStyle CWP event.
+        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
+    ///     clears the shadow views while keeping the existing <c>Margin.View</c> allocated.
+    /// </summary>
+    [Fact]
+    public void ShadowStyle_Null_After_Construction_Clears_ShadowViews_But_Keeps_MarginView ()
+    {
+        Button button = new ();
+        View marginView = button.Margin.View;
+
+        Assert.NotNull (marginView);
+        Assert.NotEmpty (marginView.SubViews);
+
+        button.ShadowStyle = null;
+
+        Assert.Null (button.ShadowStyle);
+        Assert.Same (marginView, button.Margin.View);
+        Assert.Empty (button.Margin.View.SubViews);
+
+        button.Dispose ();
+    }
+
+    /// <summary>
+    ///     Verifies that <see cref="ScrollButton"/> does not create shadow infrastructure during construction
+    ///     because its <see cref="Button.OnInitializingShadowStyle"/> override sets
+    ///     <c>args.NewValue = null</c>, eliminating the create-then-destroy allocation pattern.
+    /// </summary>
+    [Fact]
+    public void ScrollButton_OnInitializingShadowStyle_Suppresses_Shadow ()
+    {
+        ScrollButton btn = new ();
+
+        // OnInitializingShadowStyle sets args.NewValue = null
+        // → base.ShadowStyle = null is a no-op → no shadow infrastructure 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 ();
+    }
 }