Low frame rate video playback in Firefox on xwayland
For example, look at any video on YouTube. It looks like the video is playing at half of the original frame rate (like every 2nd frame is skipped). Doesn't happen on Gnome Shell (on Wayland) or wlroots-based compositors.
Tested on 5.0.0 and current master (bff27cb8). Arch Linux, kernel 4.19.0-rc7, mesa 18.2.2, xorg-server 1.20.1, firefox 62.0.3 GPU: RX Vega 64