bug#20628: 25.0.50; Incorrect line height for some fonts

[Clément Pit--Claudel]

Hi,

Some Unicode characters incorrectly increase the height of the line on which
they are inserted, by an incorrect amount (typically 4/5 times the normal line
height). This is due to Emacs falling back to a font for which line height
calculations are incorrect. The problem can generally be reproduced just by
inputing the following characters:

  (𝓝𝓟)

Alternatively, the problem can be reproduced by switching to certain
specific fonts. For example:

  (set-frame-font "-unknown-Latin Modern 
Math-normal-normal-normal-*-*-*-*-*-*-0-iso10646-1" nil nil)

This problem is not specific to 25.0.50. It is discussed on stackexchange [1]
and the effect can be observed in [2]. It only occurs with specific fonts. It
particularly impacts packages that rely on prettify-symbols-mode to display math
symbols; when users install the package, some lines in the buffer start being 4
or 5 times taller than other lines, although no characters on the affected lines
stand out. For this reason, even if this is likely a problem in the way the
fonts are packaged, it would be nice to have a workaround at the Emacs level.

Emacs is the only program on my system that displays this behaviour; typing the
same text in gedit or switching gedit to one of the misbehaving fonts, for
example, does not affect the line height.

[1] http://emacs.stackexchange.com/questions/251/
[2] 
https://cloud.githubusercontent.com/assets/2506825/7760973/67ceaaea-ffd5-11e4-8bf6-d796aa162b0e.png

---

In GNU Emacs 25.0.50.4 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2015-05-14 on c-mint
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Linux Mint 17.1 Rebecca

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB

Important settings:
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
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ietf-drums mm-util help-fns mail-prsvr mail-utils mule-util time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt
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font-lock syntax facemenu font-core frame cl-generic cham georgian
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custom widget hashtable-print-readable backquote dbusbind gfilenotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 76472 5652)
 (symbols 48 18496 0)
 (miscs 40 87 99)
 (strings 32 11421 4141)
 (string-bytes 1 314062)
 (vectors 16 9839)
 (vector-slots 8 396279 11643)
 (floats 8 102 46)
 (intervals 56 177 0)
 (buffers 976 11)
 (heap 1024 40850 1039))