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.