According to willdurand, the current Unified Extensions panel was created as a subview of the AppMenu mainly out of convenience, as it allows us to quickly stand up a new panel that is styled properly.
There are a few problems though:
- The way that the AppMenu subview panels work, is that once the panel closes, the subviews get thrown back into the AppMenu subview cache. That means if we have any overflowed addon widget nodes in there after bug 1797155 lands, then we'll need to update a number of places in CUI to get those nodes from the cache, which is kind of awkward.
- The temporary panel that gets created for AppMenu subviews ends up being a child of the nav-bar <toolbar>, which ends up having us inherit this rule: https://searchfox.org/mozilla-central/rev/59f0bf3c13dd455d9f5415b89178de701ea6b850/toolkit/content/xul.css#144-148, which hides the labels of the overflowed items. We could override that, but it feels awkward.
In general, especially considering how old the OverflowableToolbar code is, and how long it's been part of CustomizableUI, I'm inclined to make the Unified Extensions panel as close in behaviour and properties as the existing widget-overflow panel, to try to avoid weird behaviours when the panel closes and the subview goes back into the AppMenu view cache.