Revisit the use of arrows in menus/popovers

We have been traditionally adding a triangle in popovers and menus, pointing to the element that opened them.

Without rounded corners these were frequently aligned to the very end of one of the element's sides.

With rounded corners this needs to be adjusted, causing some side effects and looking odd in some cases, because it either no longer points to the right place, or it requires the menu/popover to be relocated, causing scrolls to appear.