Stop propagation of click events on the MenuButtonView dropdown toggle

Review Request #11820 — Created Sept. 19, 2021 and updated

Review Board

The dropdown toggle in the MenuButtonView splitbutton would let click
events propagate up, and depending on what child node of the toggle was
being clicked, the modal library from Djblets could interpret it as a
request to close the modal.

This is where Djblets handles the click event.

This patch stops click events on the toggle from propagating, so the
Djblets library never has a chance to get confused.

Fixes bug #4944.

Local manual testing on the Review editor publish splitbutton. Ran all
JS unit tests, and wrote a new unit test for this particular case.

Fix issue 4944.
Checks run (2 succeeded)
flake8 passed.
JSHint passed.