RELEASING the button with the + icon on it in the bottom left corner causes the playlist add menu to appear (with options Other,Directory,URL,File). PRESSING the button should popup this menu.

The existing behaviour is buggy because it means you have to PRESS and RELEASE (click) the button, then find the menu option you are looking for, then PRESS and RELEASE (click) the desirec option. The correct behaviour for a popup menu is that PRESS (button-down) pops up the menu and RELEASE (button-up) over the desired option selects it.

Note that the attached patch is backward compatible, the current behaviour continues to work (i.e. it is possible to PRESS and RELEASE the button to leave the menu in a posted state without requiring you to keep the mouse button down). This alternate behaviour is still useful if you want to slow down and study the menu options carefully or for users with motor difficulties. Notice that this alternate PRESS-RELEASE-SELECT-PRESS-RELEASE menu operating mode has always been possible with GTK+ menus (and impossible with menus in older MacOS versions), but it is a bug that the faster PRESS-SELECT-RELEASE mode does not work currently with this menu.