"A man goes to The Doctor .. So don't do that"
The same answer could apply to all bugs.
"When I click upvote it doesn't work .. Well don't vote..".
But the point here is it's not a bug, nor status-by-design, it's the way the internet works and in fact this site handles that quite well I think.
The popup box when you click "flag" is site content, and it can be moved around. The fact it can be moved outside the standard site dimensions (X or Y) could return a few things:
The box doesn't produce a horizontal or vertical scroll, as such
you "lose" part of the content (the box) out of the site into no mans land and
you cannot get to it fully. In fact you cannot get to it at all if you "throw" the flag box, thus needing the horizontal scroll to retrieve it.
The box does produce a horizontal or vertical scroll, and as such
you do not lose any part of what is actual site content as the site
page dimensions grow with you moving the box.
When you think about it, 2. is more desirable by far, and more logical that actual site content changes the page dimensions as a part of the site. Rather than just disappearing as if it's suddenly not classed as site content.
In fact, if you are moving the box out of the way so you can access site content while deliberating over which flag to choose, then it changing the page dimensions is better still, as then you can move it right out of the way without losing it, and scroll to get it back if needed.