I haven't tested extensively, but at first glance, this seems to fix my problem (though, I rarely change window size and usually use Blender maximized).

Workaround doesn't work here, either, and setting the resolution scaling to "Default for Display" does not fix the problem for me.

Still very much happening, and happening consistently on iMac Pro. Is completely unusable on this machine with just a single monitor.

Re-attached image. I'm on an iMac Pro. I'm glad it doesn't happen for you, but it happens consistently, 100% for me. The iMac Pro has some significant architectural and component differences from the other iMac models and (e.g. Xeon instead of Core I chips, Radeon Pro Vega instead of Radeon Pro 5xx).

Odd. I never turned it on. I can turn it off, but it won't "stick" when I click ctrl-u. It always turns back on when I launch, and is on by default after resetting to factory settings.

Is there possibly a setting that could possibly be impacting this? It's 100% reproducible on my machines.