The inclusion of the word "Portable" in your search is likely a confusion with the Summer series from the visual novel developer , as this animation is an adaptation of a game by that developer.
Here's an interesting essay:
A portable version would allow you to carry those animated memories in your pocket. The scene where the protagonist and heroine watch the final sunset of summer vacation—animated with soft color shifts and drifting clouds—becomes a private, repeatable experience. natsu ga owaru made natsu no owari the animation portable