When you insert a TreeView control under a TabControl, then select a TreeViewItem, then the TreeView.SelectionItemChanged event fires on every node below the node that’s being selected.
(1) Build and run the repro app
(2) Click “1” in the TreeView control This will cause SelectionChanged even to fire, which then adds the text “1” to the TextBox to the right.
(3) Press “Two” to go to the second tab in the TabControl Doing this should not trigger any SelectionChanged event to fire on TreeView.
“2” and “3” are both added to the TextBox due to SelectionChanged event was fired on all them.
(You can set a breakpoint at line38 in Page.xaml.cs file inside the SelectionItemChanged event handler to see this happening)