A brief history

• MPL's idle event gets used for performing events in the idle cycle for animation, see http://matplotlib.org/examples/event_handling/idle_and_timeout.html which gives an example of this
• This example however didn't work for GTK3 and made idle_event() in backend_bases.py return True #3769 fixed this but brought about the unintended side effect of CPU 100% usage while idling on some backends. #4092 whether or not one actually used it, because of this made idle_event() in backend_bases.py return True #3769 got reverted in Revert "made idle_event() in backend_bases.py return True" #4533.
• Since the creation of idle event, we now have matplotlib.animation so we no longer need idle events for that task, but other uses may exist (@mdboom ?).

Current Status

So we now have warning messages back on GTK3 on close and the example not working again (no change from the stable release 1.4.x).

Several discussions got started in #4092 describing possible workarounds, but also whether we should scrap idle event altogether. This issue serves to continue that now discussion now that the side effects stated in #4092 have been reverted and thus closed.