Toggling tabs-on-top breaks the urlbar-search-splitter (the resizer between the location bar and search bar, when these two are next to each other). STR: * Make sure your urlbar-search-splitter is working (try dragging it around) 1. Toggle Tabs-on-Top (either way) * the splitter becomes the very last child of nav toolbar, and moves to far right of toolbar (rendered useless) This fault stays with the window for its whole lifespan.