The dictionary shall resolve this issue. Now, what is the definition of seen, according to the New Oxford American (so, no colour) Dictionary?

past participle of see.

And the definition of see is…

1 perceive with the eyes; discern visually: in the distance she could see the blue sea | [no object] : Andrew couldn’t see out of his left eye | figurative : I can’t see into the future.
• [with clause] be or become aware of something from observation or from a written or other visual source: I see from your appraisal report that you have asked for training.
• be a spectator of (a film, game, or other entertainment); watch: I went to see King Lear at the Old Vic.
• visit (a place) for the first time: see Alaska in style.
• [in imperative] refer to (a specified source) for further information (used as a direction in a text): elements are usually classified as metals or non-metals (see chapter 11).
• experience or witness (an event or situation): I shall not live to see it | [with object and complement] : I can’t bear to see you so unhappy.
• be the time or setting of (something): the 1970s saw the beginning of a technological revolution.
• observe without being able to affect: they see their rights being taken away.
• (see something in) find good or attractive qualities in (someone): I don’t know what I see in you.

And what is a past participle?

noun Grammar
the form of a verb, typically ending in -ed in English, that is used in forming perfect and passive tenses and sometimes as an adjective, e.g., looked in have you looked? and lost in lost property.

The perfect tense requires that an action both start in the past and end in the past or to have been “completely done.” Whether the mere seeing of part of the movie means that the action is completely done is open to interpretation.