@kkh I was able to duplicate the pixelated corner doing the steps you said, I opened both Safari and Mail, and kept the Mailboxes view open with an email selected. Alt-tabbing or even clicking somewhat quickly between the two resulted in that squared off group of pixels of the Mail app at random times. Sometimes it would remain for several "cycles" but it would also sometimes go away for several more. As for the flickering of the app window itself, I can't seem to get that to happen.
Also, I tried doing the same thing in other apps like Finder windows, Messages, Maps, Photos, etc. and this corner pixelation I can only get to happen in Finder and Mail, no other apps. I was able to do this in both Default as well as the 2048 resolution I've been using for a few days now.
Are you only seeing the corner pixelation in other apps/windows?