Silverlight TextBox.TextChanged event when tab item is unfocused

Question

It's easy to test that TextChanged event won't fire when a text box is inside inactive tab item of a tab control. The same is true when text box is invisible (but unselecting tab item doesn't set text box's Visibility property to Collapsed). I guess the problem is not specific to text box or TextChanged event. What I need is to force TextChanged event even if text box is hidden. How can I achieve this?