Created attachment 556692 [details]
See testcase, with that, I get an extra taskbar entry in the Windows taskbar, with no title in there. So I guess that's from the plugin window.
Created attachment 558485 [details]
on-again off-again test case
Created attachment 558487 [details] [diff] [review]
On Windows, SetParent(nsnull) strips the child attribute and sets the parent to the desktop. To avoid it showing up on the taskbar you also need to hide the window.
Fixed by backout.