Another reviewer mentioned this. It's a good idea, but I certainly wouldn't make it a top priority if you have more interesting work. I have already wished I could undo a change. Start with one level deep, maybe go three or four if it's not too painful. The current workaround is to just manually delete the thing you just did. Undo could be limited to deleting the most recently added widget/spacer/panel/sizer, etc., or it could extend to undoing the property you just changed (more work probably ...).
That's certainly on the list.
The internal structures would not allow this easily at the moment. At the moment it would require to dump the whole file on every structural change just to be able to restore it. This might be too slow.
For now undo could only be implemented for property changes. See history.History if you want to contribute...
repository and tickets moved to GitHub: https://github.com/wxGlade/wxGlade/issues/47