Following on https://phabricator.kde.org/D23169, it turns out one can simplify the toolbuttons with menu rendering strongly by rendering the whole frame at once, and removing the code for detecting the presence of the menu alltogether. This makes for cleaner code with no visual difference with respect to how it should look (and no 'double-shadow').
Essentially this is the same patch as D23169, with more code removed, in order to avoid the rendering of the frame a second time around the arrow.