Usually that occurs if an application has created an invisible window that has the WS_VISIBLE property for whatever reason. Could you do the following to figure out which process that titlebar button thinks it's sitting on top of?

• Open the Settings > Advanced Settings window
• Scroll down to the TitleBar Buttons section
• Enable the "Show Debug Info" option
• Click OK, then OK again
• Hover your mouse over the TitleBar Button, it should show a tooltip with some extra info, including the "Path" information

If you can attach a screenshot of that tooltip, that would be great

Thanks!