Mail pops up unprompted and divides iMac screen in half while playing Amazon video

This has been described elsewhere but as yet there doesn't seem to be a solution so I thought I'd re-up. Almost every time I watch Amazon video for more than an hour or so the Mail window pops up unprompted dividing the iMac's screen in half. The Amazon left half of the screen continues to play (audio & video); the Mail right half of the screen has to be manually closed before the video returns to full screen. As dramatic suspense interruptions go it's pretty intense. Running Chrome 74.0.3729.169 on a late 2014 3.5 GHz Intel Core i5 on Mojave 10.14.5. Any advice will be most gratefully received - cheers.