Why does KeyDown event not have access to the current value of bound variable?

Your textbox->property binding is only happening after the textbox loses focus. When you type in text and press enter, you have not set the focus anywhere else on the form. You can demonstrate this by typing in your text, clicking on the scroll viewer, then clicking back on the textbox and hitting enter.

You will then see your text change in the viewer.

Does it work if you change AddLine to use TheLineTextBox. Text?

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