Move the point towards the mouse-cursor.
This will make big point moves. Mainly, set the point to the largest subterm NOT containing the current point, but containing the clicked position.
If clicked in a different window, shift focus to it.

Note: This key is also used to fill in an obref such as [ob name] with an object name.
Just put the point INSIDE the empty text slot that needs an ob name, then `(mousemiddle)' in any Nuprl window at all, and its name will be inserted into the slot. IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html