When the Undo button is disabled Ctrl-Z and Alt-Backspace still do something. When the focus is somewhere else they change focus to the panel showing the file contents.
Example
- Select two files and open the Compare contents dialog (menu Files/Compare By Content).
- Change one of the file names at the top so the panels are cleared (do not re-compare yet).
- Press Alt-E to enable/disable the option Case sensitive. This option now has focus.
Ctrl-Z, Alt-Backspace move the focus to one of the panels.