Okay I have figured it… or at least enough of a workaround to resume working.
I have two monitors. Although they are physically similar in size, resolution on my newer monitor is nearly three times as high. I usually keep this new monitor off, and it is not my primary monitor. Half the time, I have the newer monitor in a vertical position.
However, I need to make it my primary monitor when I work in Pro Motion NG because the modal windows keep popping up where I can’t reach them with the stylus.
If I keep moving my program window between the two monitors (say I needed to save/export a file really quick without any serious alterations), I may end up in a situation where the modal window pops up on the primary (low resolution) monitor with the offset from the secondary monitor (high resolution) so I cannot see the window, and I cannot access it even from Shift-right click -> Move with arrow keys panel, because of reasons you’d be more familiar with than I. This can lead to a severely broken state.
I was able to get out of it by dragging window back and forth and rearranging windows and swapping monitors until I got the modal window to display on the big screen. But, you know, a couple panic attacks later as my frantic posts have illustrated.
I think the whole bucket of pain would be avoided if the modal windows always popped up on the same screen as the program, regardless of which monitor is the primary.
I’ve also noticed that the certain container windows (brush container, view, tool and paint settings) resize themselves according to the resolution of the primary monitor, not the monitor the program is on currently, even if you try ‘Options -> Windows -> Rearrange windows’ to bring them back in line.
Phew. Hope this helps!