Bug 1920319 - add dedicated keypress handler for new tab buttons, r=tabbrowser-reviewers,dao
Right now, tabbrowser relies on browser-toolbarKeyNav.js for keyboard access to these buttons on macOS.
There's also toolkit handling for space, but not enter, in https://searchfox.org/mozilla-central/rev/9fe73403523732f57cd82d30590ecc272fb0b165/toolkit/content/widgets/button.js#27-34 .
In vertical tabs mode, browser-toolbarKeyNav does not help us as the buttons aren't in the toolbox.
On Nightly, the containers-related code in https://searchfox.org/mozilla-central/rev/9fe73403523732f57cd82d30590ecc272fb0b165/browser/components/tabbrowser/content/tabs.js#1422-1428
adds the buttons to gClickAndHoldListeners, which have their own keypress handling. This happens whenever
containers are enabled. Those are not enabled by default on non-nightly, and so we end up missing a keypress
handler in vertical tabs mode on non-Nightly on macOS.
This is all a big mess and we should probably figure out a better solution. But given this needs beta uplift,
the expedient solution seems to be a dedicated listener and making sure that only 1 listener is invoked.
Differential Revision: https://phabricator.services.mozilla.com/D224094
70debc7c0a