I have a custom control (derived from Control) that is placed on a flyout. It contains a Grid with a TabControl on it and three TabItems under that. One of the controls (a button) on the last Tab closes the unpinned flyout. Lookling at Snoop it looks like my custom control has a property called TabbedMdiHost.IsCloseButtonOnTab which is True. If I use Snoop to set it to False the flyout behaves correctly. I have been unable to figure out where this property is coming from. I've looked for TabbedMdiHost in the descendants of the visual tree, but no luck. And no XAML properties show up that look like this one. Any suggestions?