I'm on 21.0.7 and when i click somewhere near the top in the black area (or close outside the progress bar) then the main window goes small until i click the green button to maximize the window again.
The OP is reporting a problem that is SPECIFIC to using a Wacom tablet instead of a mouse. Those touch pads that you draw on with a stylus.
The behavior you are reporting seems to be the "auto un-maximize" behavior. Here's what happens for me: If the window is maximized and I click it in it's title bar area, nothing happens. Double click, same thing. Double click used to toggle maximized and non-maximized, but this was fixed several months ago. If yours still does this, you should upgrade to the latest build.
Ok, so clicking does nothing. But if I click and then drag (while holding the mouse button down) like I want to move the window, then it goes down to it's non-maximized size, whatever that happens to be. It's whatever size I last set it to when not maximized. I can then move the window. When I let go it stops but stays non-maximized. Clicking the green maximize button brings it back to maximized again.
Doesn't seem like much of a problem to me, but maybe it works differently with the older build (21.0.7). Try upgrading to the latest and see what happens.
Brian.