That element is a PaneTabItem. That is the element that provides the Tab in an UnpinnedTabArea or when in a TabGroupPane. So you would probably have to create your own Style for PaneTabItem that sets the ToolTip property. If you want it to show the ToolTip of the ContentPane then you can bind it to its Pane.ToolTip but the tooltip will also show when over the contentpane itself (e.g. when the flyout is displayed and the mouse is over the element) so you may want to create your own attached property and bind the ToolTip to that.

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