@noiob Yep. That's probably the one which has worked in the past.
@noiob Just at a guess, I need to promote the floater to a full layer, then do that...
@woozle that might be it lol
"why doesn't it let me do this thing right now" is also my GNU imp experience
@noiob I'm just kinda mystified by the design-choice of making everything outside the original image-size invisible until you do the right incantations.
...which turned out to be: "to new layer".
So, immediate problem solved. (Larger problem remains of Gimp's UX being compulsively nonintuitive.)
@woozle I'm sure there's a very good reason to allow layers to have different sizes
I can't think of any so it's probably "someone implemented it that way and no one cared enough to fix it"
@woozle I assume you already tried
Layer > Layer to Image Size
?