There are a bunch of historically grown issues with current Toolbar/Tools layout/code (in no logical order and mostly caring about WikimediaUI theme here):
[x] {T176106}
[x] Minimize action button selector output – https://gerrit.wikimedia.org/r/#/c/381800/1
[x] Remove `box-shadow` from active tool-items not in design – https://gerrit.wikimedia.org/r/#/c/381628/1
[] Fix `border` on narrow bottom toolbars overflow – https://gerrit.wikimedia.org/r/#/c/381802/1
[] PopupToolGroup header is a feature, but neither in demos nor styled – https://gerrit.wikimedia.org/r/#/c/381801/
[] Highlights (`background-color`) of active tools and toolbar menus should be equal
[] Turn more/fewer arrow in more/fewer tool in bottom positioned toolbar
[] `position: relative` to Toolgroup.less in general
[] Accellerator keys padding should be handled per theme, not in theme-independent PopupToolGroup
[] Tools and menus focus should be aligned with other widgets