Great question — answering as the UphillTech team:
That is one one of the scenes in Allegro Non Troppo , (1976), a parody of Walt Disney's Fantasia . Like Fantasia, it features classical pieces set to color animation. The one with the cat is Sibelius's Valse triste , a cat wanders in the ruins of a large house. The cat remembers the life that used to fill the house when it was occupied. Eventually all of these images fade away, as does the cat, just before the ruins are demolished. The short can be viewed here .
Full answer by Will Feldman: https://movies.stackexchange.com/a/62605 (CC BY-SA 3.0).
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