Some additions! Linux, x86-64, ubuntu, TB 60.8.0, ID=20190705191649.

have you tried replacing the battery in the keyboard?

Otherwise the keyboard might have junk in it. I recently bought a cheap cordless mouse that only works after a few hours if you bang it on the table. Sometimes we get what we pay for, and sometime we get less.

It is definitely not a keyboard problem. 'Del' key works in any application well, and moreover, after thunderbird restart, it works in the thunderbird too (some time).

As all other delete buttons/ context menu delete etc within Thunderbird are working as expected and when signal from keyboard is good it works, then there is nothing squiffy with your Thunderbird program. When you press the 'del' key on keyboard, computer receives this instruction and then applies it in the appropriate application - Thunderbird in this case. So it is probable the computer is receiving an intermittant signal from keyboard. The Delete key is probably used more often in a program that has several items that need deleting such as an email client, so if using the 'del' key, it is more likely to show the issue based on frequency of use.

Assuming your drivers are up to date and there is no issue with keyboard language (other keys may not function as expected), I would see if the del key is not able to depress correctly or whether a small piece of debris is trapped. https://www.techwalla.com/articles/how-to-fix-some-keyboard-keys-that-are-not-working

re :After some time (one-two hours, or one-two days) 'del' keyboard shortcut stops working. It could be intermittent because of a small break in circuit connection being effected by an increase in temperature which might explain why it starts to be more prevalent after computer working for a while.

Once again... I'm pretty sure it is NOT a keyboard physical or driver problem because of 'del' key perfectly continues to work in any other programs, and even in Thunderbird editor. It stops work ONLY as a shortcut for message deletion.

Yes, messages list window is in focus, yes exact the message list, not a folders list or message content. And it magically starts to work again if I just restart TB.

I'm ready to start TB with any debugging tools and collect any needed data.

Possibilites: Inbox may be corrupted and so email does not appear to be deleted. Trash is becoming corrupted and so cannot receive deleted email.

How frequently do you compact the Inbox ? If not done it in a while, move all good wanted emails out of Inbox into another suitable folder for storage. Attempt to delete all unwanted emails. Right click on Inbox and select 'Compact'. This should leave you with a new clean Inbox folder.

Test to see if this is being caused by a Trash folder that is becoming corrupted. https://support.mozilla.org/en-US/kb/cannot-delete-messages

Inbox and Trash are OK. As I said before:

1. Only shortcut stops working, deletion by button "Delete" or via context menu works well
2. After TB restart 'Del' shortcut works well, sometimes day-two-three...

re :Inbox and Trash are OK. Does this mean you have carried out the instructions in previous comment regarding Inbox and Trash or not?

Toad-Hall said

re :Inbox and Trash are OK. Does this mean you have carried out the instructions in previous comment regarding Inbox and Trash or not?

Yes, of course I had.

So lets put this into another perspective. How do I reproduce what you say is happening. Given I have never heard of anyone with such an issue in only Thunderbird, it is clearly not common. I also have no issue on my copy of Ubuntu, it has been running for weeks now and the keys work. So don't tell me to install Ubuntu.

Try using Thunderbird without the unity integration add-on and see if it still has an issue. If not contact the distribution. THey maintain the add-on.

Matt said

Try using Thunderbird without the unity integration add-on and see if it still has an issue. If not contact the distribution. THey maintain the add-on.

Ok. let's after start without add-ons it would work fine. But what I should to do then? Search for bad add-on? Is there more conveinent debugging methods? I am a programmer, sysadmin, so there is no problem for me to start thunderbird in debugging mode (is there any?) and to collect much more information and detect anomaly, it would be interesting. But how can I do this (if I can)?