Every time I CLOSE a Write tab in Thunderbird Mail, I get a persistent white square (2.5 inches on my large screen - see image below) which sits above everything else (although the cursor recognises what is hidden underneath). If I move the Mail window around the screen the square remains fixed in screen position. Several closures bring on several squares. The only solution is to reboot the PC - closing Mail doesn't work.

However, if I detach the Write tab from the Mail window (by dragging to a new window), I can close it without the problem arising. 

I have spent weeks exploring this problem with Mozilla/Thunderbird and we have finally cracked it - Write should always open in a new WINDOW (and therefore behave correctly on closure). The tab only opens because Groupy (v1.5) tells it to, as I had Thunderbird included in auto-grouping by default.

Clearly I can now leave it ungrouped and the problem goes away. But I wondered if you were familiar with this issue - and if so whether any solution was available or in prospect.

Many thanks