window-combination-limit

window-combination-limit is a built-in function in `C source code'.

(window-combination-limit WINDOW)

Return combination limit of window WINDOW.
WINDOW must be a valid window used in horizontal or vertical combination.
If the return value is nil, child windows of WINDOW can be recombined with
WINDOW's siblings. A return value of t means that child windows of
WINDOW are never (re-)combined with WINDOW's siblings.