TextBox Class
TextBox has built-in handling for the
bubbling MouseUp and MouseDown
events. Consequently, custom event
handlers that listen for MouseUp or
MouseDown events from a TextBox will
not be called. If you need to respond
to these events, listen for the
tunneling PreviewMouseUp and
PreviewMouseDown events instead, or
register the handlers with the
HandledEventsToo argument (this latter
option is only available through
code). Do not mark the event handled
unless you deliberately want to
disable TextBox native handling of
these events, and be aware that this
has notable effects on the control’s
UI.
In you code you are firing just MouseLeftButtonUp