set-window-combination-limit

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

(set-window-combination-limit WINDOW LIMIT)

Set combination limit of window WINDOW to LIMIT; return LIMIT.
WINDOW must be a valid window used in horizontal or vertical combination.
If LIMIT is nil, child windows of WINDOW can be recombined with WINDOW's
siblings. LIMIT t means that child windows of WINDOW are never
(re-)combined with WINDOW's siblings. Other values are reserved for
future use.