When minimized, TB sometimes will not open when its tray icon is clicked, and must be closed manually. When restarted, it opens in a very small window.
This has been happening for over a year. If restarted in Safe Mode, this does not ever happen ... until the next time Thunderbird restarts normally. The "very small window" is the smallest one possible for TB: I cannot make it smaller by dragging its borders. As far as I know, I'm not using any plugins, and if started without plugins but NOT in Safe Mode, the problem eventually will occur. "Eventually" may be every few days or a couple of times a day; completely unpredictable.
Other programs running while TB is minimized, leading up to the problem: Chrome (sometimes), Python/Perl scripts. But, those are running when the problem does not occur.
If TB has been running OK and is manually closed, it always reopens normally.
Chosen solution
If you have ruled out add-ons and broken layout, check that hardware acceleration is disabled in Tools/Options/Advanced/General. Are you using a non-default theme in normal mode or a userChrome.css file?
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Try this: launch TB in safe mode, check 'Reset toolbars and controls' and click 'Make changes and restart'. Any difference?
I've tried that several times in the past, and it did not solve the problem. Only "Continue in Safe Mode" avoids the problem, and that's only effective until the next time Thunderbird starts.
Chosen Solution
If you have ruled out add-ons and broken layout, check that hardware acceleration is disabled in Tools/Options/Advanced/General. Are you using a non-default theme in normal mode or a userChrome.css file?
Hardware acceleration was enabled and now is disabled. Restarted TB normally (not in Safe Mode), but with add-on's disabled and toolbars reset. Will see what happens and update this thread within a week.
Have been using the default theme and AFAIK am not using any custom css.
Thank you for your quick and helpful replies.
Turning off "Hardware acceleration" seems to have fixed the problem. Thank you very much for following up repeatedly until you identified the problem and provided the solution. :-)