Get rid of an unexpected white line above the status strip due to a changed (#12646)

* Changing the renderMode to System when the StatusStrip was selected

* Add publicAPI

* Removing the override function "Select"

* Remove StatusStrip.Select from PublicAPI.Unshipped.txtRemovinf

Author: LeafShi1

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

@@ -29,10 +29,7 @@ public StatusStrip()
         SuspendLayout();
         CanOverflow = false;
         LayoutStyle = ToolStripLayoutStyle.Table;
-
-        // Default changed for SystemColorMode from System to ManagerRenderMode.
-        // Also to be consistent to the MenuStrip.
-        RenderMode = ToolStripRenderMode.ManagerRenderMode;
+        RenderMode = ToolStripRenderMode.System;
         GripStyle = ToolStripGripStyle.Hidden;
 
         SetStyle(ControlStyles.ResizeRedraw, true);
@@ -363,6 +360,17 @@ protected override void OnLayout(LayoutEventArgs levent)
         EnsureRightToLeftGrip();
     }
 
+    internal override void ResetRenderMode()
+    {
+        RenderMode = ToolStripRenderMode.System;
+    }
+
+    internal override bool ShouldSerializeRenderMode()
+    {
+        // We should NEVER serialize custom.
+        return (RenderMode is not ToolStripRenderMode.System and not ToolStripRenderMode.Custom);
+    }
+
     internal override bool SupportsUiaProviders => true;
 
     protected override void SetDisplayedItems()

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/StatusStripTests.cs

@@ -556,18 +556,18 @@ public void StatusStrip_RenderMode_ResetValue_Success()
 
         control.RenderMode = ToolStripRenderMode.System;
         Assert.Equal(ToolStripRenderMode.System, control.RenderMode);
-        Assert.True(property.CanResetValue(control));
+        Assert.False(property.CanResetValue(control));
 
         control.Renderer = new SubToolStripRenderer();
         Assert.Equal(ToolStripRenderMode.Custom, control.RenderMode);
         Assert.False(property.CanResetValue(control));
 
         control.RenderMode = ToolStripRenderMode.ManagerRenderMode;
         Assert.Equal(ToolStripRenderMode.ManagerRenderMode, control.RenderMode);
-        Assert.False(property.CanResetValue(control));
+        Assert.True(property.CanResetValue(control));
 
         property.ResetValue(control);
-        Assert.Equal(ToolStripRenderMode.ManagerRenderMode, control.RenderMode);
+        Assert.Equal(ToolStripRenderMode.System, control.RenderMode);
         Assert.False(property.CanResetValue(control));
     }
 
@@ -584,18 +584,18 @@ public void StatusStrip_RenderMode_ShouldSerializeValue_Success()
 
         control.RenderMode = ToolStripRenderMode.System;
         Assert.Equal(ToolStripRenderMode.System, control.RenderMode);
-        Assert.True(property.ShouldSerializeValue(control));
+        Assert.False(property.ShouldSerializeValue(control));
 
         control.Renderer = new SubToolStripRenderer();
         Assert.Equal(ToolStripRenderMode.Custom, control.RenderMode);
         Assert.False(property.ShouldSerializeValue(control));
 
         control.RenderMode = ToolStripRenderMode.ManagerRenderMode;
         Assert.Equal(ToolStripRenderMode.ManagerRenderMode, control.RenderMode);
-        Assert.False(property.ShouldSerializeValue(control));
+        Assert.True(property.ShouldSerializeValue(control));
 
         property.ResetValue(control);
-        Assert.Equal(ToolStripRenderMode.ManagerRenderMode, control.RenderMode);
+        Assert.Equal(ToolStripRenderMode.System, control.RenderMode);
         Assert.False(property.ShouldSerializeValue(control));
     }