I don't know if an update to Chromium or to xf86-video-intel is causing this, but starting with this week I got some strange issues with Chromium's UI not being updated properly when "Override software rendering list" is enabled in chrome://flags/. Of course, this option is disabled by default with a reason, but this means that Aura, the new Chromium's UI, will not be hardware-accelerated and also it will cause HTML5 videos to look pixelated when scaled up or down (e.g. on YouTube).

Scaling with accelerated UI (left) and w/o accelerated UI (right)

With that option enabled I got a lot of strange issues, like text selection not being updated until I scroll the webpage or items in dropdown-boxes not being always highlighted when the mouse cursor is over them. I fixed this by switching to Intel's old acceleration method, UXA. The new SNA acceleration method is faster but newer, which means that it is prone to bugs, while UXA is more mature and stable. Don't forget to reboot the system after creating or modifying /etc/X11/xorg.conf.d/20-intel.conf file.