The venerable test-actors used the default stage, which meant that
closing the window just stopped the main loop but left the scene intact.
This does not happen with a normal stage created through
clutter_stage_new().
The change happened in 6ed6b2a54b5c8f44496399057ad5e58771bfc620, in an
unhelpfully named commit that was just supposed to switch to static
colors. My bad.
Anyway, the change led to some assertion failures when closing the stage
in the middle of an animation.
The correct thing to do when using an actor from another object (a
Timeline ::new-frame callback in this case) without owning the instance
is to connect to the ::destroy signal and clean up the pointer to avoid
calling an invalid actor.
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