https://bugzilla.mozilla.org/show_bug.cgi?id=1223198#c23 noticed a bug in GTK 3.4 which means the theming engine is often not set before a style context is used. https://bugzilla.mozilla.org/show_bug.cgi?id=1319650#c27 observed that working around this resolved bug 1222704 and bug 1269984, which became more frequent with the patches in bug 1319650.

Comment on attachment 8864644 [details] bug 1362170 force a style resolution on context creation to set GTK 3.4 theming_engine https://reviewboard.mozilla.org/r/136314/#review139576 ::: widget/gtk/WidgetStyleCache.cpp:727 (Diff revision 1) > + // That can result in incorrect drawing on first draw. To work around this, > + // force a style look-up to set |theming_engine|. It is sufficient to do > + // this only on context creation, instead of after every modification to the > + // context, because themes typically (Ambiance and oxygen-gtk, at least) set > + // the "engine" property with the '*' selector. > + if (gtk_get_minor_version() < 6) { As gtk-4.0 is comming, can we have a complete 3.6 check here?

Comment on attachment 8864644 [details] bug 1362170 force a style resolution on context creation to set GTK 3.4 theming_engine https://reviewboard.mozilla.org/r/136314/#review139576 > As gtk-4.0 is comming, can we have a complete 3.6 check here? I'm hoping 3.4 support can be removed before we need to think about 4, but I've added a check in case this does not happen. (Ubuntu 12.04 is already EOL, but bug 1217931 tracks moving Mozilla infra off this version.) Note that gtk_check_version(3, 6, 0) != nullptr would be true not only for GTK 3.4 but also 4.x, 5.x, etc. I'm assuming GTK 3 builds will not run against GTK 4. The gtk_style_context_get* method interfaces differ, for example.

Pushed by ktomlinson@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/81977c96c6ff force a style resolution on context creation to set GTK 3.4 theming_engine r=stransky+263117