fix(library): stop the shelf from hearing about the tiles it asked for itself

docker/frontend/src/engine/library.ts

@@ -1855,9 +1855,18 @@ export function onThumbWritten(listener: (id: string, blob: Blob) => void): () =
 // A tile made because someone looked at the frame, kept where the next visit
 // will find it: the decode is paid once per frame, on the visit that first drew
 // it, and the reads after that are the same reads a scanned tile would have had.
-export async function putPhotoThumb(id: string, blob: Blob): Promise<void> {
+// The tile is in hand either way; `tell` is whether anyone has to hear about it.
+//
+// The store writes this module makes for itself — the walk's previews, and the
+// tiles `makeThumb` mints for the shelf — are the shelf's own: it asked for that
+// tile and is about to draw it. Telling it about those is a stamp per tile, and a
+// stamp re-reads the window, and a re-read is the read in flight cancelled — on a
+// roll of thousands the tiles then never land at all, which is a shelf that stays
+// grey. Only a rewrite from somewhere else (the studio grading the frame the reader
+// is standing on) is worth the telling.
+async function writeThumb(id: string, blob: Blob, tell: boolean): Promise<void> {
   cacheThumb(id, blob);
-  for (const listener of thumbListeners) listener(id, blob);
+  if (tell) for (const listener of thumbListeners) listener(id, blob);
   try {
     const db = await openDb();
     if (!db.objectStoreNames.contains(THUMBS)) return;
@@ -1868,6 +1877,10 @@ export async function putPhotoThumb(id: string, blob: Blob): Promise<void> {
   }
 }
 
+export function putPhotoThumb(id: string, blob: Blob): Promise<void> {
+  return writeThumb(id, blob, true);
+}
+
 export async function getPhotoThumbs(ids: string[]): Promise<Map<string, Blob>> {
   const res = new Map<string, Blob>();
   const missing: string[] = [];
@@ -1969,7 +1982,7 @@ const inFlight = new Map<string, Promise<Blob | null>>();
 // later visit reads the tile back instead of the file.
 async function tileOf(id: string, preview: Blob): Promise<Blob> {
   const made = (await makeTile(preview)) ?? preview;
-  void putPhotoThumb(id, made);
+  void writeThumb(id, made, false);
   return made;
 }
 
@@ -2013,7 +2026,7 @@ async function makeOneThumb(photo: LibraryPhoto): Promise<Blob | null> {
       // is drawn as it is rather than written to the store whole.
       const made = await makeTile(file);
       blob = made ?? file;
-      if (made) await putPhotoThumb(photo.id, made);
+      if (made) await writeThumb(photo.id, made, false);
     }
     return blob;
   } catch {