15.7.1.1.2 No flection, if the direct object is depending on an infinitive
You migh think that things go too far now, however, there is still some condition to be taken into account: If the direct object is depending on a following infinitive, the perfect participle is not changed..
Examples
La pomme qu' elle a mangée n' était pas encore mûre.
The apple that she ate was not yet ripe.
La pomme qu' elle a décidé (nicht: décidée) d' acheter n' était pas encore mûre.
The apple that she decided to buy was not yet ripe.
