Mail.app keeps opening new view windows.

Recently (a month or two) the behaviour of my Mac OS Mail.app changed: When I select a mail message in the list column it displays correctly in the large right hand pane, but it also opens a new window displaying the message.

It's the same behaviour you get for double-clicking a message - so if you try that you can see what i mean. However, I am only single-clicking.

I wondered if my mouse was to blame, but I don't have any problem with double clicks in any other app - it only happens in mail.app & it doesn't happen 100% of the time. It happens a lot when I first use the app, but after clicking back & forth on many messages & closing the pop-ups it seems to settle down & act normally.

Mail 11.5, OS 10.13.6