SOMEWHAT IGNORE ALL THIS. Looks like this is a screen refresh issue, though maybe a bug, not the shutter bug...the search continues...
Latest thinking, bugs happen after a change in exposure settings. Believe, based on DM stuff below, that two processes may be over-stepping each other, focus and exposure?
When glitches begin "Clear all settings" in Canon menu seems to fix things.
Can someone try this? Keep autofocus on. Go into Movie mode, set camera on Manual, then change either shutter or aperture until the image would be seriously under exposed, like 1/2500 f/5.6. Then do a half-press of the shutter, maybe a few presses. My screen gets corrupted.
I then tried with 22mm. Canon "Clear Settings". Works fine. Change to Manual mode, still works. Change to grossly underexposed image, shutter bug.
Went in, enabled DM log. No bug! Tried photo mode, movie mode, nothing. Save DM log. Tried again, fine. Re-start, under-exposed, bug is back.
Put 18-55 back on. Same thing. DM logging somehow stops bug, even after it's turned off. But once camera is on again, screen corruption bug comes back from under-exposed shot.
Timing issue as Alex? suggested with memory read/write?
Misc Evidence:
o. To test potential exposure/focus overlapping, zoomed lens and focused quickly between window, floor, anything that might confuse it and screen did start to get corrupted--though was difficult to achieve.
o. If I turn OFF auto-focus and one-shot in movie menu, the screen still goes haywire, but not as badly. After setting exposure correctly, camera goes back to normal.
o. In manual focus mode, it happened that by changing the shutter the screen will go up a quarter of an inch with each change. So moving the wheel, and part of the screen moves up, and the bottom part goes white. Moving shutter speed back down brings back the image part of the screen down. Almost as if you can move the screen up an down when the wheel is changing the shutter speed.