In Caps, the 'undo' and 'redo' buttons in the sidebar don't seem to do anything when they're clicked. This is consistent in any web browser I've tested with (Chrome, Safari and Firefox on macOS 13.0, as well as Chrome and Edge on Windows 11). Using undo/redo via keyboard shortcuts behaves as expected.
Looking into it further, it doesn't look like any part of the Caps JavaScript bundle references these button elements to register click events on them.
Adding the following JS code would fix this:
bind('#button-undo', 'click', undo);
bind('#button-redo', 'click', redo);