I've been facing a really bizarre problem on my 15" 2011 MBP running El Capitan over the past couple days.
Sometimes - possibly more often, or exclusively, under high load - some apps will suddenly no longer react to key presses entered in text areas and input fields, for example:
- in the URL bar in Safari
- in all input fields when writing an E-Mail in Mail.App
- in Spotlight's search bar
However, hotkeys (e.g. Cmd+Q to quit the app) still work!
Also, when I switch to a different app, keyboard input will work there as expected. Then when I go back to the app with the problem, it'll be broken again.
This never happens in all apps at once - right now, for example, Spotlight doesn't accept any input, but the browser with which I'm writing this question works (obviously).
This is not a hardware problem: it occurs on both my Bluetooth Apple keyboard, and the MBP's built-in keyboard.
Quitting the application in question will help for a couple of minutes, then it'll happen again. Restarting the machine doesn't help.
I'm not seeing any weird processes or anything in the process list.
What could be causing this?