The picture below tells the story; that's the whole browser window. There is a vertical scrollbar but no horizontal one. I have this zoomed in (as in, normally, in the browser, it snaps to at 150%), so the width fills the screen (you can see where Home is top left), but I think the problem is it is way off center and cannot be moved.1
Notice the moderator flag interface in the bottom third; this happens when that "Close question" button is pressed; if I load the page normally, the normal close button and dialogue work properly.
This is from Google Chrome 76 on GNU/Linux. Here's an view that isn't zoomed which provides a clue about the issue:
Whereas the base Stack Exchange interface is statically sized, the flag handling interface is dynamically sized and grows to fit regardless of zoom -- i.e., it is not off edge in either picture the way the popup is (it is harder to tell with the first one because there is not the right/left nav bumpers since there are no further flags, but I did double check this). Possibly the popup, when invoked from that context, uses an incorrect value for the screen geometry.
I didn't think to try un-zooming (forgot I use it) until I had lost the screen and won't get a chance until there's another flag, but I still think the "way off center" is wrong.
I also didn't realize it can be moved by mousing over the top area of the popup until informed in comments here.
left: calc(50% - 400px);
in the style of the modal. For the "normal" close button on the page, this is 400px to the left of the center of the screen, as it should be. However, the button in the lower pane creates the modal relative to this pane, where apparentlycalc(50%)
does not correspond to the center of the screen, but instead to some position to the left of the center, which results in the modal being created too far to the left of the screen.