I use ProtonMail app on my Apple MacBook.

The 'Trash' icon should NEVER result in the deletion of all emails in a thread where previous thread items were archived.

At the VERY LEAST a pop-up should warn the user that all items in the thread will be deleted if I proceed and give an option to back out.

I have had numerous occasions where I accidentally 'Trash-ed' an email associated with a thread, from my Inbox and the entire thread was trashed (even though all other items in the thread were previously archived). If there was at least a pop-up to warn that all email in the thread will be trashed, I would have a chance to back out.

Initial reaction might be 'just go to Trash and recover'. However, I am one of those users that actually empties my Trash on a regular basis... and I don't always take the time to review what is being emptied (nor should I have to).