If a DOM rebuild was scheduled on a previous frame but the rebuild is no longer needed (e.g. due to the "list all tabs" menu no longer being open) then do not clean up nor populate the DOM.
In bug 1953533 we debounced and delayed full DOM rebuilds of the "list all tabs" menu due to severe performance impacts during event-heavy operations like restoring an entire browser session.
When the "list all tabs" menu is closed, it's supposed to get emptied and stop responding to events. However, it was possible to perform actions in the "list all tabs" menu that would trigger events and close the menu at the same time. The triggered event would schedule a "list all tabs" menu rebuild on the next frame, which would then build the "list all tabs" menu DOM despite the menu being closed. When the menu was reopened, the menu builds the menu DOM by inserting new DOM elements, but since the menu was already built, you end up with a doubled version of the tab strip.
Differential Revision: https://phabricator.services.mozilla.com/D246047
eacb38eda0