window-combination-resize

window-combination-resize is a variable defined in `C source code'.
Its value is

nil

Documentation:
If t, resize window combinations proportionally.
If this variable is nil, splitting a window gets the entire screen space
for displaying the new window from the window to split. Deleting and
resizing a window preferably resizes one adjacent window only.

If this variable is t, splitting a window tries to get the space
proportionally from all windows in the same combination. This also
allows to split a window that is otherwise too small or of fixed size.
Resizing and deleting a window proportionally resize all windows in the
same combination.

Other values are reserved for future use.

This variable takes no effect if the variable `window-combination-limit' is
non-nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.