So my iPhone died and was replaced. iPad and Mac still had Signal message history.
Signal on new phone asked to use iPad to restore, but QR code thing didn’t fully work.
Then on iPad, there’s a “relink with phone” button. Hit that, it immediately wipes history and proceeds from there. No warning, no apparent way to restore.
(Just in case anyone from Signal is reading: many of these screens also lack a “Cancel” button. Have to kill and restart app.)