It's a violation.
9.2.5. Play: Thrower A1 inadvertently steps through the plane of the boundary line and touches the court inbounds. A1 immediately steps back into normal out-of-bounds throw-in position. The contact with the court was during a situation: (a) with; or (b) without defensive pressure on the throw-in team.
Ruling: A violation in both (a) and (b).
Comment: Whether or not there was defensive pressure or whether or not stepping on the court was inadvertent, it is a violation and no judgment is required in making the call.
Contrary to what you may have read, a thrower can't dribble the ball inbounds either.
