The previous commit inserted the round-media MultiDataTrigger INSIDE a <Setter> instead of into the
Style.Triggers collection, so every window that loads PreviewPane threw
"System.Windows.MultiDataTrigger is not of type System.Windows.SetterBase" at parse time. A Debug
build would have failed to open the main window at all.
Caught by `AlertBoxPreviewVisibilityTests` and `AboutHubTests` in the suite, which is the reason
those UI tests exist. Both green again, as is `MediaPreviewDisplayTests`.