mouse-pixel-position

mouse-pixel-position is a built-in function in `C source code'.

(mouse-pixel-position)

Return a list (FRAME X . Y) giving the current mouse frame and position.
The position is given in pixel units, where (0, 0) is the
upper-left corner of the frame, X is the horizontal offset, and Y is
the vertical offset.
If Emacs is running on a mouseless terminal or hasn't been programmed
to read the mouse position, it returns the selected frame for FRAME
and nil for X and Y.