Why GTK_THEME env breaks Adwaita applications?

[andre1 (Andre)]

Why GTK_THEME enviroment variable breaks Adwaita applications?

[ebassi (Emmanuele Bassi)]

Because libadwaita contains its own style, and overriding it will cause breakage onto unsuspecting libraries and applications.

The GTK_THEME environment variable is only meant as a debugging tool, not as user configuration; see:

• Gtk – 4.0: Running and debugging GTK Applications
• Gtk – 4.0: Running and debugging GTK Applications

[alicem (Alice Mikhaylenko)]

Yeah, we’ll start unsetting GTK_THEME from libadwaita in future, there’s a lot of confusion around it:

• How to let GtkMenuButton be flat using GtkBuilder?
• GTK_THEME breaks adw_style_manager_set_color_scheme ADW_COLOR_SCHEME_PREFER_DARK (#677) · Issues · GNOME / libadwaita · GitLab
• numerous times in Matrix
• etc.

Some apps already unset it: for example: build-aux/org.gnome.World.PikaBackup.Devel.json · main · World / Pika Backup · GitLab

[andre1 (Andre)]

Ok, but since the applications using Libadwaita, why is GTK_THEME overwriting?

[andre1 (Andre)]

I think shouldn’t do it. The problem is with old installations and will resolve itself.

[ebassi (Emmanuele Bassi)]

@andre1

Ok, but since the applications using Libadwaita, why is GTK_THEME overwriting?

GTK uses CSS, which (as the name implies) cascades through multiple layers.

Adwaita is using GTK, and GTK_THEME overrides the system theme; so the changes that Adwaita imposes on widgets will not work any more.

@andre1

I think shouldn’t do it. The problem is with old installations and will resolve itself.

No, it won’t resolve itself, unless GTK stops honouring the GTK_THEME environment variable, or breaks its semantics.

[andre1 (Andre)]

@ebassi

Adwaita is using GTK, and GTK_THEME overrides the system theme; so the changes that Adwaita imposes on widgets will not work any more.

But if:
GTK use by default theme Default from hard coded path (or is this also configurated with GTK_THEME?)
GTK_THEME setups different path
Libadwaita sets GTK_THEME variable

How GTK_THEME can override Libadwaita stylesheet if Lbadwaita always sets its own end of the chain?

[alicem (Alice Mikhaylenko)]

Libadwaita doesn’t use GTK_THEME for this, no. In fact, its styles aren’t even a theme.

[andre1 (Andre)]

Then, can you explain, how Libadwaita works? IMy understanding is that it’s just a library that make new more complicated elements/widgets from GTK elements/widgets and it contains a stylesheet that controls colors and shapes (theme).

Is this how GTK works:

@andre1

GTK use by default theme Default from hard coded path (or is this also configurated with GTK_THEME?)
GTK_THEME setups different path

[alicem (Alice Mikhaylenko)]

Libadwaita? It sets gtk-theme-name (not GTK_THEME) to an empty theme and loads styles using Gtk.StyleContext.add_provider_for_display

GTK_THEME overrides gtk-theme-name and breaks everything.

GTK_THEME is a debug variable, it’s not configuration and never was that.