The text insert cursor appears when a control has the keyboard focus. The diagram control calls Focus() on the edit field when you double click a node to edit its text. However the keyboard focus might not change if an event handler for events such as PreviewGotKeyboardFocus and PreviewLostKeyboardFocus sets e.Handled. So, check if some controls from the window that contains your auxiliary diagram don't handle these events. You could also try invoking e.TextBox.Focus() yourself from EnterInplaceEditMode event handler, but there's no reason your call to work better than ours

I hope that helps,
Stoyan

I suppose you could toggle scroll viewer's focusable state in response to Enter/Exit InplaceEditMode events then, if you also need keyboard scrolling.

I hope that helps,
Stoyan