With passive futiḥa, al-bābu is the nominative deputy subject (nāʾib fāʿil), not an accusative direct object.
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Qad Before a Passive Verb
Qad: Expectation, Confirmation and Contextual Meanings
With passive futiḥa, al-bābu is the nominative deputy subject (nāʾib fāʿil), not an accusative direct object.