Bug#880674: libpango-1.0-0: Thai word break stops working since 1.40.13

[Theppitak Karoonboonyanan]

Package: libpango-1.0-0
Version: 1.40.13-1
Severity: important

Dear Maintainer,

Since Pango 1.40.13, Thai word break appears to be broken. This affects
all GTK+-based text editors like gedit, mousepad, leafpad, etc. as well as
Mozilla Firefox.

Try, for example, opening this page with Firefox:

  https://linux.thai.net/~thep/text/aphaimanee.html

The long continuous-text paragraph is supposed to be wrapped, but it's not.

Downgrading Pango to 1.40.12 does solve the problem.

Looking at upstream repository, this commit looks like the culprit:

  https://git.gnome.org/browse/pango/commit/?id=c4619480e536e393e2d4a8e26a6ceb5af1fe80e3

I've tried writing a simple program to test it, and it appears
PangoLogAttr::is_char_break for all Thai characters except the first
one of the line are cleared to zero, which causes break_thai() in
pango/break-thai.c to skip setting is_line_break in all positions.

-- System Information:
Debian Release: buster/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable'), (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.13.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=th_TH.utf8, LC_CTYPE=th_TH.utf8 (charmap=UTF-8), LANGUAGE=th_TH.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages libpango-1.0-0 depends on:
ii  fontconfig    2.12.3-0.2
ii  libc6         2.24-17
ii  libglib2.0-0  2.54.2-1
ii  libthai0      0.1.27-1

libpango-1.0-0 recommends no packages.

libpango-1.0-0 suggests no packages.

-- no debconf information