Add a warning about UI animations.

.github/workflows/ci.yml

@@ -195,7 +195,9 @@ jobs:
 
           - backend: linux
             runs-on: ubuntu-latest
-            pre-command: "sudo apt-get update -y && sudo apt-get install -y python3-gi python3-gi-cairo gir1.2-gtk-3.0 python3-dev libgirepository1.0-dev libcairo2-dev pkg-config libfuse2"
+            pre-command: |
+              sudo apt-get update -y && sudo apt-get install -y python3-gi python3-gi-cairo gir1.2-gtk-3.0 python3-dev libgirepository1.0-dev libcairo2-dev pkg-config libfuse2
+              gsettings set org.gnome.desktop.interface enable-animations false
             briefcase-run-prefix: 'xvfb-run -a -s "-screen 0 2048x1536x24"'
 
           - backend: windows

gtk/tests_backend/widgets/base.py

@@ -1,8 +1,9 @@
 import asyncio
+import warnings
 
 from pytest import skip
 
-from toga_gtk.libs import Gtk
+from toga_gtk.libs import Gio, Gtk
 
 from .properties import toga_color, toga_font
 
@@ -14,6 +15,16 @@ def __init__(self, widget):
         self.native = widget._impl.native
         assert isinstance(self.native, self.native_class)
 
+        # Check if GTK UI animations are enabled.
+        settings = Gio.Settings(schema="org.gnome.desktop.interface")
+        if settings.get_boolean("enable-animations"):
+            warnings.warn(
+                "GTK UI animations are enabled; any tests failures are likely "
+                "caused by this. Run `gsettings set org.gnome.desktop.interface "
+                "enable-animations false` to disable animations.",
+                RuntimeWarning,
+            )
+
     def assert_container(self, container):
         container_native = container._impl.container
         for control in container_native.get_children():