The actor allocation will be invalid on CLUTTER_TOUCH_BEGIN, because
it comes together with a CLUTTER_ENTER event that will recalculate
styles, and queue a relayout in result.
The net result is that on CLUTTER_TOUCH_BEGIN, the relayout has been
already queued, so the slider width comes up as 0, and the value ends
up as 1. Later touch events already happen on a validated actor, so
it is corrected. Still, not fun when modifying the volume slider on a
touchscreen.
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