Does one who smiles at a scene mocking religion, despite inwardly resisting it and their heart disapproving of the mocker, leave the fold of Islam and have their deeds nullified?

The default state is for you to remain Muslim, and the certainty of faith does not vanish merely due to doubt. Your aversion to mockery of God is evidence of your faith. Therefore, turn away from obsessive thoughts and do not expose yourself to sin by watching content that mocks God and His verses. If someone apostatizes from faith and then repents, their deeds prior to apostasy are, according to the more preponderant view, not nullified, because the nullification of deeds is restricted to dying in a state of disbelief, as stated by the Almighty: "And whoever of you reverts from his religion and dies while he is a disbeliever - then those have had their deeds nullified in this world and the Hereafter, and those are the companions of the Fire; they will abide therein eternally." Thus, the absolute verses regarding the nullification of deeds by apostasy are qualified by dying in a state of disbelief.