Prioritize split along the longest edge by default.

Currently, `split-window-sensibly' prefers to try to split
vertically first, disregarding the actual shape of the frame
or the user preferences.  This is a good default when Emacs
is taller than wider.  However, when Emacs is in full-screen
(landscape screen layout), trying to split vertically may not
be what the user expected, since there is plenty of space
available on the right.

Typical scenario: Emacs is in landscape layout, one buffer is
open in a window covering the entire frame.  Another buffer is
opened in a second window (C-x 4 f). Both splits are feasible
but users may prefer the horizontal one.

This patch preserves the behavior of the `split-height-threshold'
and `split-width-threshold' variables.  Splitting continues not
to be permitted if the edge length is below the threshold.

* lisp/window.el (split-window-sensibly): First tried split
direction follows user preferences.

* etc/NEWS: Add an entry for new variable
`split-window-preferred-direction'.
* doc/emacs/windows.texi: Document new variable.

1 files changed, 4 insertions, 1 deletions

diff --git a/doc/emacs/windows.texi b/doc/emacs/windows.texi
index a992f26fcdd..a4941435073 100644
--- a/doc/emacs/windows.texi
+++ b/doc/emacs/windows.texi
@@ -511,6 +511,7 @@ selected frame, and display the buffer in that new window.
 
 @vindex split-height-threshold
 @vindex split-width-threshold
+@vindex split-window-preferred-direction
 The split can be either vertical or horizontal, depending on the
 variables @code{split-height-threshold} and
 @code{split-width-threshold}.  These variables should have integer
@@ -519,7 +520,9 @@ window's height, the split puts the new window below.  Otherwise, if
 @code{split-width-threshold} is smaller than the window's width, the
 split puts the new window on the right.  If neither condition holds,
 Emacs tries to split so that the new window is below---but only if the
-window was not split before (to avoid excessive splitting).
+window was not split before (to avoid excessive splitting).  Whether
+Emacs tries first to split vertically or horizontally, is
+determined by the value of @code{split-window-preferred-direction}.
 
 @item
 Otherwise, display the buffer in a window previously showing it.