Function: window--major-non-side-window

Return the major non-side window of frame FRAME.
The optional argument FRAME must be a live frame and defaults to
the selected one.

If FRAME has at least one side window, the major non-side window
is either an internal non-side window such that all other
non-side windows on FRAME descend from it, or the single live
non-side window of FRAME. If FRAME has no side windows, return its root window.