Appearance regression for "inline-toolbar" styled GtkToolbar with more than 2 packed GtkToolButtonItems

[Andrew Chadwick]

Xenial's Amibience and Radiance themes make GtkToolbar widgets with 3 or more packed GtkToolButtonItems packed into them look strange if the toolbar has the "inline-toolbar" style (GTK_STYLE_CLASS_INLINE_TOOLBAR). There are screenshots available at this downstream bug: https://github.com/mypaint/mypaint/issues/684

Clusters of widgets like these are seen in the MyPaint layers panel (hi, developer here), and can be observed with gtk3-demo (CSS Theming → Style Classes). They're a common way of making small groups of list tools available.

The whole point of inline-toolbar as I understand it is to make toolbars of 2 *or more* tool items join up nicely. Xenial has rearranged a lot of these toolbars into pairs, and I respect that for usability reasons. However this regression seems like a bug to me. Not all groups of related actions can be clustered into pairs! I *don't* think we're abusing GTK, since this pattern looks just fine in Debian GNOME under Adwaita from GTK+ 3.20.4 (gnome-themes-standard 3.20).

Nobody from the Ambience/Radiane team bothered to respond to https://answers.launchpad.net/ubuntu/+source/light-themes/+question/294023 so I am assuming it's a bug. See the linked question for code to reproduce the issue.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.