fix: Border of SplitButton of ToolStrip is not contrasting enough … (#12092)

* fix: Border of SplitButton of ToolStrip is not contrasting enough in High Contrast theme #12083

Author: Epica3055

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStripHighContrastRenderer.cs

@@ -146,8 +146,9 @@ protected override void OnRenderSplitButtonBackground(ToolStripItemRenderEventAr
             else if (item.Selected)
             {
                 g.FillRectangle(SystemBrushes.Highlight, bounds);
+                g.DrawRectangle(SystemPens.HighlightText, dropDownRect);
+
                 DrawHightContrastDashedBorder(g, e.Item);
-                g.DrawRectangle(SystemPens.ButtonHighlight, dropDownRect);
             }
 
             Color arrowColor = item.Selected && !item.Pressed ? SystemColors.HighlightText : SystemColors.ControlText;

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStripSystemRenderer.cs

@@ -575,6 +575,17 @@ protected override void OnRenderSplitButtonBackground(ToolStripItemRenderEventAr
             {
                 DrawArrow(new ToolStripArrowRenderEventArgs(g, splitButton, splitButton.DropDownButtonBounds, arrowColor, ArrowDirection.Down));
             }
+
+            ToolBarState state = GetToolBarState(e.Item);
+            if (e.Item is ToolStripSplitButton item && !SystemInformation.HighContrast &&
+                (state == ToolBarState.Hot || state == ToolBarState.Pressed || state == ToolBarState.Checked))
+            {
+                var clientBounds = item.ClientBounds;
+                bounds.Height -= 1;
+                ControlPaint.DrawBorderSimple(g, clientBounds, SystemColors.Highlight);
+                using var brush = SystemColors.Highlight.GetCachedSolidBrushScope();
+                g.FillRectangle(brush, item.SplitterBounds);
+            }
         }
         else
         {

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolstripProfessionalRenderer.cs

@@ -296,7 +296,10 @@ protected override void OnRenderSplitButtonBackground(ToolStripItemRenderEventAr
 
         if (buttonPressedOrSelected && !item.Pressed)
         {
-            using var brush = ColorTable.ButtonSelectedBorder.GetCachedSolidBrushScope();
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+            using var brush = Application.IsDarkModeEnabled ?
+                Color.Silver.GetCachedSolidBrushScope() : ColorTable.ButtonSelectedBorder.GetCachedSolidBrushScope();
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
             g.FillRectangle(brush, item.SplitterBounds);
         }
 

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripButtonTests.Rendering.cs

@@ -162,8 +162,9 @@ public void ToolStripButton_SplitButton_Selected_RendersBackgroundCorrectly_High
                 bounds: null,
                 points: null,
                 State.Brush(SystemColors.Highlight, BRUSH_STYLE.BS_SOLID)),
+           Validate.SkipType(ENHANCED_METAFILE_RECORD_TYPE.EMR_POLYGON16),
            Validate.Repeat(Validate.SkipType(ENHANCED_METAFILE_RECORD_TYPE.EMR_POLYPOLYGON16), 2),
-           Validate.Repeat(Validate.SkipType(ENHANCED_METAFILE_RECORD_TYPE.EMR_POLYGON16), 2));
+           Validate.SkipType(ENHANCED_METAFILE_RECORD_TYPE.EMR_POLYGON16));
     }
 
     private class ToolStripSystemHighContrastRenderer : ToolStripSystemRenderer