[Bug 1846699] Re: Missing mechanism in Glibmm that prevents the support of some themes (CSS) in Gtkmm

[Sebastien Bacher]

Deleting the bionic target for now, it's unclear it's important enough
for a SRU yet and it's missing a testcase which could be used to verify
an update

** No longer affects: glibmm2.4 (Ubuntu Bionic)

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https://bugs.launchpad.net/bugs/1846699

Title:
  Missing mechanism in Glibmm that prevents the support of some themes
  (CSS) in Gtkmm

Status in glibmm2.4:
  Unknown
Status in glibmm2.4 package in Ubuntu:
  Fix Released

Bug description:
  In  GTK+ 3.20 the theming mechanics was heavily changed.

  https://developer.gnome.org/gtk3/stable/ch32s10.html

  One of the major changes was the use of element names much more than
  style classes. For this the function gtk_widget_class_set_css_name()
  was added to t he API and used to assign the element names. However,
  this was not added in GTKmm, making impossible to assign element names
  in custom widget, and no automatically assignment was developed in
  GTKmm's Widget class in order to be able to identify widget classes in
  the theming system (CSS).

  The problem was discussed and tracked up to Glibmm in GNOME bug
  tracking system:

  https://gitlab.gnome.org/GNOME/gtkmm/issues/17
  https://gitlab.gnome.org/GNOME/glibmm/issues/33

  The issues are due to the lack of the mechanism to call
  gtk_widget_class_set_css_name() function in the instance init
  function.

  This was solved in Glibmm2.4 2.60, however Ubuntu is still behind this
  version and it's affecting some of our products.

  We have applied a patch to the current version (2.56) of Glibmm2.4 in
  Ubuntu 18.04 successfully, but I would like to ask you to consider
  upgrading the version of Glibmm in the official repository or apply
  the patch.

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