It seems I have lots of questions that either aren't explained anywhere, or that I don't know the right search term to find them! This time, I'm not sure how one is supposed to remove an instant preview block from the LyX editor.
I put an instant-preview block around an entire float block, but apparently it can't preview that (which makes sense) so I put one inside the float block, which does work, but now my float block is surrounded by a black border in the editor, and I'd like to get rid of that (especially because something about this arrangement of multiple nested previews crashed LyX once earlier, and I'd like it not to crash again). I found the "dissolve inset" button in the context menu, but that only seems to get rid of the float block, which I want to keep.