I believe it stays locked in horz sync. I can still see the full raster, but its dimensions change.
I thought it used to shrink to a dot. But I just captured this morning's fade-out (after 7 minutes) with my phone, and actually it only shrinks to about half the normal horiz width before the image fades out.
Here's what it looks like:
https://www.dropbox.com/scl/fi/3qnl8...u20f9oyxc&dl=0