Patch just generally improves the experience in the mediawiki theme with buttons that aren't direct children of the toolbar but are in a buttongroup. This seems to cover menuToolGroup, but more importantly popupToolGroup which is what I think is actually needed for the dependent tasks of [ Save | v ]. I added a new Toolbar demo, showing that case, since it seems like something people might want to replicate.
I can see the argument for making something like a ButtonDropdownWidget which automates all that creation, if we think it's a common case.
I think a major issue here is that the dropdown is a different colour to the primary 'save' button:
That's pretty much just that PopupToolGroup doesn't mixin Flaggable. (And then if it did, I think the flaggable styles are tightly scoped to ButtonElements, so more theming changes would be needed. Or making it also mixin ButtonElement, I guess.)