bug#14825: 24.3.50; split-window-below miscounts window lines

[Eli Zaretskii]

To reproduce:

 emacs -Q
 Click S-mouse-1 and select "Change Buffer Font"
 Under "Size" type 6 and click OK

You should now have *scratch* buffer displayed with a very small font

 Hit RET 5 times to have a 10-line buffer
 C-x 2
 C-u 10 C-x ^

You should now have 2 windows displaying *scratch*, the lower one about
10 lines tall.

 C-x o
 C-x 2
  => Window #<window 0x3817df0 on *scratch*> too small for splitting

But that window is 10 lines tall, while the minimum window height is 4
lines, so I would expect Emacs to honor the request and produce 2
windows.  And indeed, if I do the same with a 10-line window without
making the font smaller, the window is split.

The reason for this seems to be that window.el and window.c do all
calculations in canonical lines, which is incorrect when the font of the
default face changes.  Moreover, the documentation barely hints on the
fact that "lines" actually means "canonical-height lines" in almost all
window-* functions that deal with vertical dimensions.



In GNU Emacs 24.3.50.32 (i686-pc-mingw32)
 of 2013-07-08 on HOME-C4E4A596F7
Bzr revision: 113326 jan.h.d@swipnet.se-20130708132613-wcxq7gxkq12qvhp3
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --prefix=/d/usr --enable-checking CFLAGS=-O0 -gdwarf-2 -g3
 CPPFLAGS=-DGLYPH_DEBUG=1'

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1255
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  buffer-face-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
<return> <return> <return> <return> <return> <S-down-mouse-1> 
C-x 2 C-u 1 0 C-x ^ C-x o C-x 2 C-x o M-x r e p o r 
t - e m <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Buffer-Face mode enabled
byte-code: Window #<window 0x3817df0 on *scratch*> too small for splitting

Load-path shadows:
None found.

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