OK, purge/reinstall of grub-pc and grub-common first seems to have done it, although I couldn't find a diff in the conffiles that should have made that work.
And then finding a close-enough theme because all the old ones are broken, same as always.
And removing the three different screensavers it installed because that became a real mess. And LibreOffice.
The install CD for this thing was 10.04. *Almost* reaching Windows' age for upgrades.
For some reason it pops up a notification that "StickyKeys is enabled" on startup, even though it isn't, and is also disabled, but that might be something to do with it having three different input mechanisms, because ibus is back to curse us again. It works, I'm not pulling that thread right now.
I also don't want to know mysql-common is apparently a very low level Jenga block that wants to install Unity desktop instead if removed, but you do you, Xubuntu.
(And I still don't understand how this update resets my random-on-login wallpaper to read from some other custom folder. Losing all the *other* settings so it's once-off and ugly-stretched is one thing, but resetting to a non-default previous value from years ago is a real mystery.)
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