Total screen freeze in El Capitan, but audio keeps playing Lately I've been having lots of issues with my Mac randomly freezing. As in, multiple times per week. These are the symptoms:


My screen freezes completely. The picture stays on screen and my computer doesn't respond to mouse or keyboard input.
If I have audio playing in the background, sometimes it will keep playing.
The Force Quit hotkey doesn't seem to do anything.
The only way out I've found so far is to force my computer off by holding Power.


How do I diagnose and fix the underlying issue?

Additional info


MacBook Air (13-inch, Early 2015)
4 GB 1600 MHz DDR3
27.34 GB free disk space (SSD)
Running apps when I notice this: Safari, Mail (there are others, but I'm not sure if they're running every time it happens so I didn't include them)
Activity Monitor screenshot: I can't, because I never know when the system is about to freeze up


As advised in the comments, I've disabled 'Allow WebGL' for now and I'll continue monitoring the situation.
If you still have another device around, you might try to SSH into your machine, as apparently only the GUI process is frozen. You'll be able to safely reboot your Mac using sudo reboot now.
Another hint on this is that the shutdown dialog still shows up if you press the shutdown button for 2-3 seconds.
This might also be a hint to the solution - it should suffice to restart the corresponding process, even though I don't know which one that is. Maybe someone with a little more expertise on the OS might be able to help here.