Three things, each a failure the thumbnail path could have caused. First, correctness: Coil keys caches by the request, and a request built from a File is keyed by its path, so a picture replaced or edited outside the app kept showing its previous self - in a file manager that reads as data loss. core/ThumbnailKey now carries path, size and modification time, is built from the numbers the listing already has (no stat, so it stays off the main thread's disk work), and has four tests that name the two ways a file changes. The request carries it as the memory and disk cache key.
Second, the placeholder: the row drew an empty square while a thumbnail decoded, which a thousand-row folder makes very visible. The kind icon is now drawn first and the picture paints over it, so every row shows what it is immediately and the thumbnail replaces it without a flicker.
Third, the cache is decided rather than inherited: memory only, at 15% of the heap, and no disk cache at all - the sources are local files that decode cheaply, and a disk copy is a second copy that survives the file changing. Coil's default would have kept up to 2% of free space of stale thumbnails.
Measured on the phone and recorded in EDGE_CASE_TESTING: a 6200-file folder holding 300 real JPEGs opened and scrolled five times with no crash, no OOM and text rows keeping their icons (PSS 86.6 MB), and a folder of 300 real JPEGs where every row drew its own picture and eight swipes down and one back left PSS at 149.6 MB with the cache full and stable. Stated as not measured: jank on this screen (this device reports no frame durations) and a 20000-image folder.
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