Provide a way to quickly preview different choices from drop-down menus (like blending modes)

When choosing blending modes (Darken, Multiply, etc.) I often don't quite know which option I want, so I pop open the menu, choose something, pop open the menu, choose something else, pop open the menu, etc. And I do this same sort of thing with other menus throughout Blender. It's tedious, especially if I want to cycle through all the available options (try it, you'll see why)

It would be great to be able to preview a choice while keeping the menu open. For instance, I could pop open the menu, move my cursor over a menu item, and press the Tab key to temporarily "choose" that item, and the scene would reflect my choice. But the menu is still open, and my choice has not been committed. I can try another item, and another, with the mouse or the arrow keys, until I find what I want.

Then I'd press Enter to commit, or Esc to cancel.