— Function: window-at x y &optional frame

This function returns the window containing the specified cursor position in the frame frame. The coordinates x and y are measured in characters and count from the top left corner of the frame. If they are out of range, window-at returns nil.

If you omit frame, the selected frame is used.

— Function: coordinates-in-window-p coordinates window

This function checks whether a particular frame position falls within the window window.

The argument coordinates is a cons cell of the form (x . y). The coordinates x and y are measured in characters, and count from the top left corner of the screen or frame.

The value returned by coordinates-in-window-p is non-nil if the coordinates are inside window. The value also indicates what part of the window the position is in, as follows:

(relx . rely)

The coordinates are inside window. The numbers relx and rely are the equivalent window-relative coordinates for the specified position, counting from 0 at the top left corner of the window.

mode-line

The coordinates are in the mode line of window.

header-line

The coordinates are in the header line of window.

vertical-line

The coordinates are in the vertical line between window and its neighbor to the right. This value occurs only if the window doesn't have a scroll bar; positions in a scroll bar are considered outside the window for these purposes.

left-fringe

right-fringe

The coordinates are in the left or right fringe of the window.

left-margin

right-margin

The coordinates are in the left or right margin of the window.

nil

The coordinates are not in any part of window.

The function coordinates-in-window-p does not require a frame as argument because it always uses the frame that window is on.