It's a bug in the "system" renderer, details in this bug report.
Microsoft's response gives a very easy workaround:
1) Create a subclass of ToolStripSystemRenderer
, overriding OnRenderToolStripBorder
and making it a no-op:
public class MySR : ToolStripSystemRenderer
{
public MySR() { }
protected override void OnRenderToolStripBorder(ToolStripRenderEventArgs e)
{
//base.OnRenderToolStripBorder(e);
}
}
2) Use that renderer for your toolstrip. The renderer must be assigned after any assignment to the toolstrip's RenderMode property or it will be overwritten with a reference to a System.Windows.Forms renderer.
toolStrip3.Renderer = new MySR();