UNDO DOES NOT UNDO! Accidentally setting height of group (instead of a component *in* group) PERMANENTLY destroys layout of group.
I accidentally changed the height of a group instead of changing the height of the *background* of the group due to a simple selection error.
Naturally, the contents of the entire group were scaled accordingly. This did not surprise me.
What did surprise me is that "undo" does not undo the damage. It reverts the height of the group, but all the ancillary damage — resizing and repositioning dozens of components in several nested groups — is NOT undone. That portion of my prototype is irretrievably destroyed.
To recover, I had to close my prototype, losing all the changes I'd made to the mockup since the last time I saved. Whatever other changes I may have made before that fateful error are lost.
Undo MUST actually undo, or you might as well not even have it.