This is a new issue.
If you edit a document, then clear a link field, then click to add a new link, a warning pops up saying you're about to leave the editor, asking if you want to save or discard.
If you hit discard the link selector does appear, but with two columns instead of one.
In case it's relevant, I'm using Firefox. This particular link field is in a repeating area of a slice, and accepts only document links, of one particular type.
(off-topic: please relax the settings of this forum to allow mkv and mp4 uploads)