Fix some issues with `window-divider-mode'

* lisp/frame.el (window-divider-default-places): New option.
(window-divider-mode): Remove option.
(window-divider-mode): Make it a "regular" minor mode.
(window-divider-width-valid-p): Drop frame- prefix.
(window-divider-mode-apply): New argument ENABLE.  Drop frame-
prefix.  Handle `window-divider-default-places'.
(frame--window-divider-mode-set-and-apply): Remove.
(window-divider-default-bottom-width)
(window-divider-default-right-width): Drop :group entries.
* lisp/menu-bar.el (menu-bar-bottom-and-right-window-divider)
(menu-bar-right-window-divider, menu-bar-bottom-window-divider)
(menu-bar-no-window-divider): Set `window-divider-default-places'
and call `window-divider-mode'.
* doc/emacs/frames.texi (Window Dividers): Document
`window-divider-default-places'.

1 files changed, 9 insertions, 6 deletions

diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index 8d24fc5806a..4a193dd6870 100644
--- a/doc/emacs/frames.texi
+++ b/doc/emacs/frames.texi
@@ -1011,13 +1011,16 @@ scroll bar height, change the @code{scroll-bar-height} frame parameter
 separate windows visually.  Window dividers are bars that can be dragged
 with the mouse, thus allowing to easily resize adjacent windows.
 
-@vindex window-divider-mode
-  To control the display of window dividers, customize the variable
-@code{window-divider-mode}.  Its value should be either
+@findex window-divider-mode
+  To toggle the display of window dividers, use the command @kbd{M-x
+window-divider-mode}.
+
+@vindex window-divider-default-places
+  To customize where dividers should appear, use the option
+@code{window-divider-default-places}.  Its value should be either
 @code{bottom-only} (to show dividers only on the bottom of windows),
-@code{right-only} (to show dividers only on the right of windows),
-@code{t} (to show them on the bottom and on the right) or @code{nil} (to
-disable window dividers).
+@code{right-only} (to show dividers only on the right of windows), or
+@code{t} (to show them on the bottom and on the right).
 
 @vindex window-divider-default-bottom-width
 @vindex window-divider-default-right-width