If you view a suggested edit which brings up the dialog to "Approve, reject, or improve edits suggested by users." and click on the reject button, you'll get a second dialog overlaying the first with a list of possible rejection reasons, with each dialog window having an X in the upper right corner:
However if you click (accidentally or intentionally) the X in the upper right corner of the "Approve, reject, or improve edits suggested by users." dialog in the background, the list of reasons stays on the screen, while the background dialog disappears, leaving the list visible on top of the question:
This appears to be a bug, and clicking the X in the background dialog's upper right corner should probably also close the list of reasons dialog, just as clicking outside of the dialogs does.