title: UWP: When are controls declared final/sealed?
body: <p>What factors determine whether the MS developers disallow inheritance from a control? (I'm assuming there are some common underlying reasons?)</p>
  <p>Perusing the Windows.UI.Xaml.Controls namespace, I don't see any mention or obvious pattern as to why some controls are declared with the <code>final</code>/<code>sealed</code> specifier while others aren't. Certainly, someone, somewhere must have wanted to derive from, say, TextBlock, only to be disappointed. But why forbid this behavior? What makes a control like Image final but not MediaTransportControls?</p>