Does the problem go away with Help > Troubleshoot Mode?

Thank you Wayne for responding. Sorry, don't know yet, because today I can't reproduce the problem, even without going to Troubleshoot Mode! All I can do is wait for the problem to recur and try to recall exactly what I did before.

Well it just happened again, and scenario was this: Inbox had several unread messages. The top one I deleted unread by clicking the Delete button on the top strip. The next message I wanted to read, so I pressed Enter and it, too, was deleted. I rescued it from the Deleted folder, but when I pointed to it now and pressed Enter, it opened as it should.

So, it's difficult to set up a repeatable experiment and I haven't been able to checkout any effect from Troubleshoot Mode.

In case it makes any difference, I am running TB 115.4.1 under Ubuntu 20.04.6

So it sounds like a delete is repeated on the message which should have opened?

When you say "clicked delete button" which part of the screen are you referring to? See the description at https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova

I believe I clicked on the Delete button in the Unified Toolbar.

OK, the following is completely repeatable: In my TB window, the Unified Toolbar is a strip at the top, below that is the Menu bar. Beneath, a left-hand pane displays all my folders while the right-hand pane displays Inbox. 1) In my Inbox I create two trivial test messages, test1 & test2, by sending them to myself 2) With Inbox listing by Date, test2 shows at the to, test1 immediately next 3) I select the top one, test2, by clicking on it 4) I hit the Delete button in Unified Toolbar; test2 disappears and test1 becomes highlighted (selected) 5) I then hit the Enter key on my keyboard; test1 also disappears 6) Both messages are found in the Deleted folder and can be dragged back into Inbox to repeat the test. So 'testX disappears' above means it is moved to the Deleted folder, as expected.

If at step 4 I hit the Delete key on my keyboard, the behaviour is different: at step 5 test1 does not disappear, it opens, which is the expected and desired action.

None of the behaviour depends on whether the messages are marked as Unread or not.

I turn on Troubleshoot Mode using Help menu, and allow TB to restart. The window appearance is now completely different, and in particular, the Unified Toolbar is not present. I do not know how to view it in order to repeat the test.

So is there any prospect of this being fixed?

So is there any prospect of this being fixed?

Can you file a bug report at https://bugzilla.mozilla.org/enter_bug.cgi?product=Thunderbird ?

I was able to reproduce on Windows. Not on a Mac running development build.