Please start Thunderbird in Troubleshoot Mode - https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does problem change?

Please start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Does problem change?

The problem went away when I started Thunderbird in safe mode, came back when I turned safe mode off. That suggests a problem with some extension or other but I don't use any.

It could also be caused by hardware acceleration. See https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird for details.

I haven't used hardware acceleration is quite some time. More trouble than it is worth.

Is there any solution to this other than reverting to a previous version of Thunderbird? Not being able to read email seems like a fatal flaw in a email program.

Is there any solution to this other than reverting to a previous version of Thunderbird? Not being able to read email seems like a fatal flaw in a email program.

https://support.mozilla.org/en-US/questions/1475881 suggests maybe a similar problem with Firefox/

Anyway, please try the following: 1) Help > Troubleshooting Information > click "clear cache" in the top right. 2) Help > Troubleshoot Mode > when you get the confirmation dialog mark both checkboxes for making the changes permanent > then click "make changes and restart".

BTW, that question is also me. And it resolved after a Firefox update. But updating Thunderbird hasn't had a noticeable effect.

updating Thunderbird hasn't had a noticeable effect.

Updating how? Following the advice made on 4/14?

pjcamp said

updating Thunderbird hasn't had a noticeable effect.

Updating how? Following the advice made on 4/14?

I did that. It didn't work. When I say I updated it, what I mean is following the update button on the About screen.

Reading this you say you have not used hardware acceleration for some time, but the reality is that there is no correct settings for hardware acceleration. For some combinations of hardware and driver revisions having it on fixes the weird artifacts in screen. At other times having it on causes them. But troubleshooting mode is with it explicitly disabled.

I lost an argument years ago to simply turn it off by default in Thunderbird because apparently Firefox has it on by default the off state is simply not tested at all in their development of the Mozilla platform of which it is part. Hence Thunderbird defaults to on. So the bottom line is, we who do not have it on are essentially using untested code, so weird results are perhaps becoming more likely over time.

That the issue was fixed in safe mode begs the question as to what exactly did occur assuming your version is not to Micrisift store version then I would look as CSS next.

Is your version from the windows store? This installer/version has some weird behaviors because it tries to be two things at once and doers it rather poorly. Especially it appears when trying to work within the sandbox created for it to run in. I think more testing by Mozilla and Mzla is needed on this. I do not recommend the windows store version to anyone.

The second item to consider is custom layout settings via CSS files in the profile. Check that the following setting in the Config Editor is set to false. toolkit.legacyUserProfileCustomizations.stylesheets If it is set to true set it to false and restart Thunderbird.

Matt: thank you for your detailed reply. I was just trying it, toggleing acceleration off and on, and it had no effect.

When Thunderbird was part of Mozilla, back in the 90's I think, I downloaded it directly from there. Ever since, I have used the internal updater. I did not get it from the windows store and I try not to get much of anything from there.

I looked at that setting and it is already set to false.

I am considering uninstalling and reinstalling but that seems like a chore that I don't have time for until a couple of weeks from now.

This is a bit of an omnibus, and I just stopped because it was getting confusing. so there are also some additional issues with your settings that could be looked at.

I would not expect uninstall and reinstalling to do more than consume your time. Although it is more than capable of making things worse by loosing your existing profile of mail accounts and contacts. None of the Thunderbird settings are changed in that process, except the system integration ones like send a file to email in file manager. Thunderbird makes almost no use of the registry and therefore there is basically nothing to refresh in a reinstall. You would do better to clear the cache in troubleshooting information (it is on the help menu). If you want a complete refresh, you can try creating a new profile. (Use the troubleshooting information and click on the about : profiles entry to create a new profile

There are really only four major things that troubleshooting mode changes.

1. It prevents the forced install of addons into Thunderbird by some antivirus products without reference to the user, Thunderbird addon manager or courtesy. These addons are just more code injections like we used to have when malware infected browsers and changed search providers etc. They only have settings in the antivirus product and generally relate to scam and spam neither of which I have seen an antivirus product be any good with. So what antivirus do you use? They also blocks all normal addons.
2. It turns the hardware acceleration to off as it is generally the less problematic of the two options and the reverse of the default.
3. It forces the menus and toolbar into their default configuration
4. It blocks user UI settings through CSS and javascript. I know you checked the preference, but try the user profile as well just to be sure. Access the Troubleshooting Information on the help menu and click on the show button for the profile. in the profile folder do you have a chrome folder?

While in the troubleshooting information scroll down to Security Software What information is given for

• Antivirus
• Antispyware

next under Legacy User Stylesheets what settings are shown?

Just to be sure, I suggest you validate that the calabri font is Ok in Windows. I do not really expect an issue here as the display works ok in safe mode.

After all the above I had an additional thought just after clicking Post.

Check in the config editor that the setting layout.css.devPixelsPerPx is set to -1.0 (default) when that is not set to it's default value everything can go messy like you have.

Thanks. I'll try these things when I get home.

As far as antivirus, I use Microsoft Defender and do a scan with Malwarebytes periodically.

One other possibly irrelevant data point: moving the mouse around the Thunderbird window causes the garbled text to move around.

Looking forward to your response. I attempted to find a relevant Firefox graphics bug but was not successful.