>  But maybe you should tell us more about the case where you need such a
property -
> `split-window' is just the Emacs paradigm for creating a new window adjacent
to an
> existing one.

​The Treemacs package displays a full-frame height window at
the left of the frame that can be used to browse directories
in an outline fashion.  It has some features that make it
attractive at times beyond just using Dired (it also can be
used in tandem with Dired).

Treemacs uses treemacs-mode for its buffers.  So I want any
window with a buffer in that mode to be non-splittable since
it is narrow and it doesn't make sense to split it in either
dimension.

Thanks for the initial pointers.  Martin's pointers sound
promising.

Bob