library: the walk position belongs to the origin, so the app beside the browser carries a reading on instead of listing the roll again

docker/frontend/src/engine/library.ts

@@ -361,10 +361,13 @@ export interface ScanProgress {
 // the catalogue only holds the frames that were read — so the reading would
 // start again at the top of the roll and walk every folder a second time.
 //
-// Session storage holds exactly that position, and holds it because it is the
-// tab's: the tab that reloads carries on from the frame it stopped at, and a new
-// tab starts a reading of its own from the top. The key goes when the reading
-// finishes.
+// Local storage holds exactly that position, and holds it for the whole origin,
+// which is the point: the tab that reloads carries on from the frame it stopped
+// at, and so does the next context to open the app — the installed one beside the
+// browser, where session storage, being the tab's own, handed it nothing and the
+// walk started at the top of the roll a second time, every folder listed again
+// and every frame it already held read again only to be skipped. The key goes
+// when the reading finishes.
 const WALK_KEY = (folder: string) => `recipescam.library.walk.${folder}`;
 
 interface WalkSaved {
@@ -383,8 +386,14 @@ interface WalkSaved {
 }
 
 function loadWalk(folder: string): WalkSaved | null {
+  // One reading at a time: a peer that has announced this folder is walking it
+  // right now, and what it has written down is a step of a walk still in
+  // progress. Taken up here, two readings would write the same roll down at
+  // once, each undoing the other's queue. In the tab's own storage this could not
+  // come up; now that the position is the origin's, it can.
+  if (peer?.folder === folder) return null;
   try {
-    const raw = sessionStorage.getItem(WALK_KEY(folder));
+    const raw = localStorage.getItem(WALK_KEY(folder));
     if (!raw) return null;
     const saved = JSON.parse(raw) as WalkSaved;
     return Array.isArray(saved?.walked) &&
@@ -410,7 +419,7 @@ function saveWalk(
   progress: ScanProgress
 ): void {
   try {
-    sessionStorage.setItem(WALK_KEY(folder), JSON.stringify({ walked, pending, frames, restored, progress } satisfies WalkSaved));
+    localStorage.setItem(WALK_KEY(folder), JSON.stringify({ walked, pending, frames, restored, progress } satisfies WalkSaved));
   } catch {
     // Nothing to write down: the reading starts from the top, as it did before.
   }
@@ -418,7 +427,7 @@ function saveWalk(
 
 function clearWalk(folder: string): void {
   try {
-    sessionStorage.removeItem(WALK_KEY(folder));
+    localStorage.removeItem(WALK_KEY(folder));
   } catch {
     // Nothing to clear.
   }