@socal lurker and
@JamesL I have sympathy with both points of view.
If you just look at the diagram, you would say this 'must be' penalised.
If you just read the text of the Law as a piece of English language.....
'preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision'
... you can argue both parts of the sentence have to be true for it to be an offence, so if you don't think the GK has been prevented from being able to play the ball, there is no offence.
The slight ambiguity with the wording is that you could also argue that a striker sat in the keeper's lap prevents him from playing the ball wherever it is and that is sufficient to meet the test