Does the rectangle appear in safe/troubleshoot mode (hold Shift when launching TB)?

I can't answer that, because it is NEVER there when I start a session, and appears unpredictably - perhaps 1-2 times a week. I should also have mentioned that closing Thunderbird leaves the rectangle on screen.

In Settings/General, bottom of page, is hardware acceleration enabled? If it is, disable it, restart TB, and see if the rectangle continues to appear.

Should I do that BEFORE an occurrence, or wait until it happens?

Disable acceleration now, restart TB, and see if the rectangle happens again. If it doesn't happen again after a week, report the outcome here.

It's done.

I didn't have to wait long - it's back this afternoon. I checked that hardware acceleration is still off.

Perhaps of interest, it popped up exactly as I closed a 'Write' tab, via the X in the top corner.

Although that certainly isn't uniquely associated with the issue, I wonder if it helps with diagnosis?

I look forward to further thoughts.

Try this: hold Shift when launching TB, check 'Reset toolbars & controls', 'Make changes & restart'.

Do you have any add-ons?

I've done that - but the rectangle didn't go away. I guess I should reboot and do it again?

Extensions enabled: Mail Merge Manually Sort Folders Message Header Toolbar customize

Themes enabled: System theme - auto

Dictionaries enabled: British English (Marco Pinto)

I see that 3 of these have available updates, but I've held back from installing them yet.

Disable the add-ons, except the dictionary, and see if it makes a difference. I would suspect the header customize add-on, although it might just be a matter of doing the updates.

OK - that's done (I couldn't disable the Theme).

I found that the updates had happened automatically.

I'll let you know what happens.

OK - the rectangle is back!

This time it AGAIN happened as I was closing an unwanted 'write' tab. If not a coincidence (see above), that should lead to the cause being identifiable.

What happens with all add-ons disabled? You could try with a fresh profile: Help/More Troubleshooting, about:profiles, to create and manage profiles. Add an account and nothing else, and test for the rectangle. The original profile is unaffected.

Try also running Windows in safe mode to test for possible graphics driver issues.

I've had no problem for a week - until today.

So (with all add-ons disabled*) I tried opening a new profile, and the problem was exactly as before - the white rectangle (I've measured it - it's actually a 2.75" square (on my large screen)) appears almost every time I close a 'Write' tab using the top corner X. Multiple squares are possible with multiple tab closures.

However, I have also established that if I detach the Write tab by dragging it away from the browser window, I can close it in the same way with no problems.

Happy Christmas

• I noticed in the Troubleshooting page, that the various search engines are also listed as enabled extensions.

Still think running Windows in safe mode is worth trying, as the issue still occurs in a new profile.

I've never seen an example of a search engine extension causing issues. Extension issues, when they occur, are almost always caused by other types of add-ons.

Not sure what you mean by a Write tab; new Write messages only open in new windows, not tabs.

That's interesting. When i click on 'Write' a new 'browser' tab appears alongside Inbox - Mozilla Thunderbird called Write (no subject) - Thunderbird. Note that this is ABOVE the message tabs (which are hidden when this is active) and the toolbars (see image below). If this is not the standard practice, I wonder if it is because I have Groupy installed - which groups like windows in any application type together as tabs. Perhaps that's the source of the problem.....

Yes, that's not standard, so it's probably due to Groupy. If Groupy is in the Startup apps, running Windows in safe mode will bypass it.

By excluding T'bird from auto-grouping, Write opens only new windows and they can be closed without a problem. So Groupy is the source of the problem.

I can leave it ungrouped - or I may follow up with Stardock to see whether this is a known issue.

Thanks for your patience.

FYI, Stardock have confirmed that they can reproduce this behaviour with Thunderbird - and have suggested a workround.

Glad it's not just me!

Happy Christmas