On mobile: multiple top and bottom tool/nav bars that automatically show/hide themselves when you scroll. They’re invariably more irritating than if they were just pinned at the top of the page (or perhaps viewport, but ideally page - I can scroll to the top of I want it back)
On desktop: animations tied to scrolling.
Anywhere: any kind of popup, modal, etc that I didn’t click on something to get. Please fuck alllllllll the way off.
I know, but those techniques are more likely to cause selection weirdness than flexbox/etc, which is why I mention them specifically.