The setMainView method of PanelMultiView controls the "mainview" attribute, which is already set or removed later in the showSubView method. When called at construction time, it changes the mainViewId if there is at least one child in the "panelmultiview" element and the original mainViewId does not already reference the first child. This would be incorrect, but in practice it never happens for either ephemeral or static panels, and can be avoided for the throw-away activated page action panel.
The setMainView method of PanelUI is never called, and this makes the corresponding PanelMultiView method removable.
MozReview-Commit-ID: 5bNidHfKFTA
01051cc093