1981760 - Ensure the pinned tab periphery element is always last in the pinned tabs container

Status: RESOLVED DUPLICATE of bug 1981766

(Firefox :: Tabbed Browser, task, P1)

Description

[Stephen Thompson [:sthompson]]

Discussion from :dao in https://phabricator.services.mozilla.com/D260156:

Is #pinned-tabs-container-periphery guaranteed to come last? I guess it isn't since pinTab just uses native appendChild, whereas we have this for the unpinned container: https://searchfox.org/mozilla-central/rev/f6a806c38c459e0e0d797d264ca0e8ad46005105/browser/components/tabbrowser/content/tabs.js#1947,1955-1958

Seems like it's possible for #updateTabStylesOnDrag to create the pinned tabs periphery element but then for other code to place a tab after the pinned tabs periphery element. That would violate the invariant we have right now that the pinned tabs periphery element is supposed to be the last element of the pinned tabs container. We should address that.