[Bug 2006756] [NEW] composite emoji shows extra vertical color lines

[Jeremy Bícha]

Public bug reported:

Impact
------
Composite emoji (such as 🫱🏾 [Rightwards Hand: Medium-Dark Skin Tone]) are not rendered correctly. By "composite emoji", I mean emoji that are made up of multiple Unicode characters joined by "ZWJ". Composite emoji are used for skin tones, gender, flags, and a few other emoji.

The incorrect rendering can be seen in Ubuntu 22.04's default text
editor, gedit, as extra vertical red and black lines after the
character. Screenshot attached.

Test Case
---------
Install the update
Close Text Editor if it is open
Open Text Editor
Paste this character in Text Editor:
🫱🏾
It should display correctly without any extra vertical lines after it.

Other Info
----------
This was originally noticed while verifying https://launchpad.net/bugs/1990677

What Could Go Wrong
-------------------
This is a one line fix cherry-picked from pango 1.50.8

pango is a text rendering library in GTK. GTK is used for most of the
apps in the default install for Ubuntu Desktop and several other Ubuntu
desktop flavors. pango is also used by mutter which is a key component
of GNOME Shell which provides the basic desktop environment for default
Ubuntu. A critical enough bug in pango could make the desktop and apps
unusuable because text wouldn't be readable. This is mitigated through
build tests and autopkgtests.

** Affects: pango1.0 (Ubuntu)
     Importance: Low
         Status: Fix Released

** Affects: pango1.0 (Ubuntu Jammy)
     Importance: Low
         Status: In Progress


** Tags: jammy

** Attachment added: "red-black-line-emoji.png"
   https://bugs.launchpad.net/bugs/2006756/+attachment/5645929/+files/red-black-line-emoji.png

** Also affects: pango1.0 (Ubuntu Jammy)
   Importance: Undecided
       Status: New

** Changed in: pango1.0 (Ubuntu Jammy)
       Status: New => In Progress

** Changed in: pango1.0 (Ubuntu Jammy)
   Importance: Undecided => Low

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https://bugs.launchpad.net/bugs/2006756

Title:
  composite emoji shows extra vertical color lines

Status in pango1.0 package in Ubuntu:
  Fix Released
Status in pango1.0 source package in Jammy:
  In Progress

Bug description:
  Impact
  ------
  Composite emoji (such as 🫱🏾 [Rightwards Hand: Medium-Dark Skin Tone]) are not rendered correctly. By "composite emoji", I mean emoji that are made up of multiple Unicode characters joined by "ZWJ". Composite emoji are used for skin tones, gender, flags, and a few other emoji.

  The incorrect rendering can be seen in Ubuntu 22.04's default text
  editor, gedit, as extra vertical red and black lines after the
  character. Screenshot attached.

  Test Case
  ---------
  Install the update
  Close Text Editor if it is open
  Open Text Editor
  Paste this character in Text Editor:
  🫱🏾
  It should display correctly without any extra vertical lines after it.

  Other Info
  ----------
  This was originally noticed while verifying https://launchpad.net/bugs/1990677

  What Could Go Wrong
  -------------------
  This is a one line fix cherry-picked from pango 1.50.8

  pango is a text rendering library in GTK. GTK is used for most of the
  apps in the default install for Ubuntu Desktop and several other
  Ubuntu desktop flavors. pango is also used by mutter which is a key
  component of GNOME Shell which provides the basic desktop environment
  for default Ubuntu. A critical enough bug in pango could make the
  desktop and apps unusuable because text wouldn't be readable. This is
  mitigated through build tests and autopkgtests.

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