I propose to remove classical menu at the top of the screen and add additional "expand buttons" to the each label. Click on this button will popup corresponding menu list applicable for given "label".
Please sorry for ugly mockup. But I hope it explains the idea.
imho the triangular selector-thing is a good idea and i would appreciate it to be used instead of the slides(left) -scrollbar. it requires less space and looks cleaner and in some way more familiar than that bars. the delta shape could stick at pages.