When choosing a large string text from the ComboBox was overlapping other controls at left side so for fixing this bug I've set the MaxWidth property and also I've added a tooltip showing the text (this will be useful when we have a large string).

Adding a new TextBlock that when has a large text will wrap the text show at the end the chars: "..."

Fixing TextBlock Style

Adding property in the Model so we can control the TextBlock Visibility.

Removed unused using

Added more comments for clarifying the use of the IsVisibleDropDownTextBlock  property.
Also I've added the Dispose() method for unsubscribing event handlers (although I'm not so sure if we are disposing this class correctly).