Workaround -- Consign this to Outer 'Space'
Create an extra 'throw-away' virtual screen for the dirty rectangle in Spaces, and just don't go there.
This tag corner only seems to appear in the Spaces screen where you started up Parallels.
I generally keep my email, chat, and a few other things that I like to flip back to and quickly take stock of, open in their own 'Space'. I lay them out to allow for big text, so they end up totally covering the screen.
So I make the totally occupied screen be that flawed one and cover up the strange piece of Windows with windows.
