I've kinda noticed this on and off for a while, and I have a workaround, but this time around it caused enough of a flipout that it's time I try and figure out what is going on.
Display adapter is a Radeon r9 200 series, drivers are version 14.12
Monitor 1 is my primary, on HDMI
Monitor 2 is secondary, where I do things like browsing, on DVI
For whatever reason, when I have hardware acceleration turned on in firefox, youtubing on monitor 2 has a tendency to cause intermittent tearing on monitor 1, especially when scrolling up or down a page. Having just done an OS reinstall, my clean firefox install had hardware acceleration on. It seemed to be fine at first, but clicking into a video from the search page blackscreened (edit: as in it was outputting a black screen, not a no input blackscreen) monitor 1, and when I tweaked the monitor's display settings to try resetting its input, it was just a solid mass of nonstop tearing until I rebooted. Monitor is displaying just fine now, and with FF's hardware acceleration off, I can youtube just fine. I'd just really like to know what the hell would be causing that.