You misunderstood what he was asking.
He wasn't asking you to switch to a different monitor. He was asking if you had any idea what triggered the issue. Both of us suspect it may have something to do with a resolution change in Windows that occurs while MC is running (and, I'd assume, Standard View is open). What is your display setup? Do you have a KVM? Does the cable route through anything else that might present Windows with a different, especially severely limited, resolution when something happens?
I've seen this kind of behavior with a variety of switches and receivers. When the display is on, the switch is on, etc, Windows detects the proper resolution. But when you switch away, instead of providing an intelligent "dummy" EDID to the computer on that port, it just "cuts it" (or worse, shows up as a monitor, but a 640x360 one). I've also seen behavior like this, commonly, with lower-end projectors, and even when some TVs go to sleep. In any case, this kind of thing forces all active elements on your screen to dramatically resize, and then resize and redraw again when you switch back.
MC should recover properly. But... It could be broken in certain cases. I've seen programs freak out before, and I think I've even seen MC freak out before in these kinds of conditions (not this precisely, but I know I've had it get "lost" off the edge of monitors before with sudden resolution changes).
In any case, we're trying to figure out the trigger. My other ideas would have to do with automation things you're doing on the machine, odd (and probably font, screen element size, etc) display customizations you might have made in Windows.
If you uninstall and remove all settings (make a Library backup first, of course). Does it go away then when you reinstall? I guess it is nearly impossible to diagnose when you can't "fix" it again reliably.
And, just to make sure... If you get the little "top-bottom arrow type" then you should be able to click and drag the divider bar to the right, and resize it bigger. Don't try to drag the arrows themselves, just anywhere on the bar. The directions of the Arrows should be irrelevant, you can always drag it to re-open it.