On Tue, Dec 5, 2017 at 3:53 AM, martin rudalics <rudalics@gmx.at> wrote:

> > ​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.
>
> You might consider two opportunities here: If the treemacs window is
> "frame bound", then it's probably best to make it a side window so that
> C-x 1 does not delete it and it does not become the sole window of the
> frame.

​​

​It is frame-bound.  I tested display-buffer-in-side-window
and that did the trick.  Thanks.  Is set-window-dedicate-p
also needed or does the side window setup take care of that?

If you don't want delete-other-windows to delete this window,
then also add:

(set-window-parameter (selected-window) 'no-delete-other-windows t)

Bob