Does the problem remain in safe mode (hold Shift when you launch TB)?

Helpful that your tried, thanks. But the shift-launch didn't solve it.

If you select a message, then Ctrl+A, are all the messages selected? If you select a message, then hold Shift and use the up/down arrow keys, can you select multiple messages?

Yes to CTRL-A but no to shift-scroll, it just scrolls to the next message. This isn't really important, a first-world whine, but I do miss the old function.

I think there must be something wrong with your input devices, keyboard, mouse or touchpad, as this function is present in every version I've used, and there have been no similar reports (and there would have been several).

If you want to try it in a clean profile, create a new one with Profile Manager, add an account, and see what happens with selections. To start PM, close TB, Windowskey+R, type thunderbird.exe -p Your current profile is unaffected, and can be started as before from PM.

Thanks, I'll try this when I have more focus in case something goes wrong.

On Nov 8th I upgraded from v68.2.1 to 68.2.2 and the problem went away.