Replace saved copies that fail to play, reject truncated saves, and re-check the device cache when the app returns to the foreground

frontend/opfs.js

@@ -85,6 +85,29 @@
     },
 
     // Revoke an object URL previously returned by getFileUrl().
+    // 'hevc' | 'avc' | null for a stored mp4. Server copies are faststart, so
+    // the sample description (hvc1/hev1 vs avc1) sits in the first megabyte.
+    async sniffCodec(videoId) {
+      try {
+        const found = await findHandle(videoId);
+        if (!found) return null;
+        const file = await found[0].getFile();
+        const head = new Uint8Array(await file.slice(0, 1024 * 1024).arrayBuffer());
+        const has = (tag) => {
+          const [a, b, c, d] = [...tag].map((ch) => ch.charCodeAt(0));
+          for (let i = 0; i + 3 < head.length; i++) {
+            if (head[i] === a && head[i + 1] === b && head[i + 2] === c && head[i + 3] === d) return true;
+          }
+          return false;
+        };
+        if (has('hvc1') || has('hev1')) return 'hevc';
+        if (has('avc1')) return 'avc';
+        return null;
+      } catch {
+        return null;
+      }
+    },
+
     revokeUrl(url) {
       if (url && _blobUrls.has(url)) {
         URL.revokeObjectURL(url);
@@ -153,6 +176,13 @@
             await writable.abort();
             throw err;
           }
+          // A connection that ends early can close the body cleanly, leaving
+          // a truncated file that would be badged "cached" yet never play.
+          const expected = Number(response.headers.get('content-length'));
+          if (expected > 0) {
+            const got = (await tmpHandle.getFile()).size;
+            if (got !== expected) throw new Error(`download cut short (${got} of ${expected} bytes)`);
+          }
         } else {
           // No createWritable on this browser — main-thread OPFS writes are
           // impossible (sync access handles are worker-only). Surface a real
@@ -187,7 +217,9 @@
     },
 
     // List all cached videos — returns [{ id, size, name }]
-    async listVideos() {
+    // strict: rethrow instead of returning a partial list, for callers that
+    // treat a missing entry as "the file is gone".
+    async listVideos({ strict = false } = {}) {
       const dir = await getRoot();
       const items = [];
       try {
@@ -200,7 +232,7 @@
           const file = await handle.getFile();
           items.push({ id, name, size: file.size });
         }
-      } catch { /* OPFS not available */ }
+      } catch (err) { if (strict) throw err; /* OPFS not available */ }
       return items;
     },