bug#22009: PATCH: Use `window-total-width' in `window-splittable-p'

[Eli Zaretskii]

> From: Joost Kremers <joostkremers@fastmail.fm>
> Date: Fri, 27 Nov 2015 02:16:22 +0100
> 
> The problem I'd like to solve (and the reason I submitted the patch) is
> that if a window has wide margins that are used to center the text,
> `window-splittable-p' will say that the window cannot be split
> horizontally, even though it can, because the margins are disposable.
> 
> But actually, that's not the real problem. The real problem is that
> `window-splittable-p' simply has too little information to make an
> informed decision.
> 
> The point is that the margins may be used for two different kinds of
> purposes:
> 
> (1) The margins can be used to display useful information, in which case
> they should be retained when the window is split.
> 
> (2) The margins can be used to reduce the width of the text area. In
> this case, the margins are disposable.

There's one more factor here: whether the margins will remain at their
width after splitting.  The mode which you presented and where the
current behavior was a problem changed the margins width as result of
the split (AFAIU), which is something the split decision cannot
possibly know.  So I don't see how we could solve the problem even in
principle if we want to handle such modes correctly.  The only way is
to ask the modes themselves, or provide a hook for them to override
the decision.

> We were discussing just such a proposal on emacs-devel, so perhaps it
> would be best to continue the discussion there?

Probably.  And I still think such changes are not for the release
branch.

Thanks.