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Choosing another row in the column read the whole roll back out of the store and asked it for a tile of every frame in one transaction: 20 000 requests in flight at once at 40 000 frames, a 200 ms block of the main thread per switch, and a mint job over the whole folder. The rows are the ones the screen is holding already, the store is asked a chunk at a time, and a folder opening under the pointer is the head of the list — the wall mints the rest as it scrolls. MAKE THUMBS by hand still walks the whole folder.