"I'd hazard a guess that you are talking about yellow layer boundary.
Check the lower half of the Layer menu for a command to bring layer's
size to image's size after you enlarged canvas."
I tried that, and the "marching ants indicating the center (original)
image stayed, and more such ants were added to the entire widened
image. Commands that were grayed out were still unavailable.
Don't think I can attach images here, so here's a link to a screen
capture on a file hosting server ...
gimp-user-list mailing list