Show context menus at the mouse position & don't cache renders

Added mouse-offset parameters. Now, the menu appears at the exact mouse position.

I'm not sure what the importance of popup-menus is. Maybe it's just a kind of render memoization, build-popup doesn't seem expensive enough to justify it.

Removed this pattern. Such memoization doesn't really work here, since the new mouse-offset tends to be a different value every time.