Just refresh the focus index when navigating chrome://media-app.

This will need more intelligence later to allow nearby files to preload.
For now it's enough to only validate the file being navigated to.

Bug: b/159757240
Change-Id: I2edfe421af38163afcea9d8bc38b138e75d11f9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262093
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781739}

chromeos/components/media_app_ui/resources/js/launch.js

@@ -306,17 +306,25 @@ async function sendFilesToGuest() {
  */
 async function sendSnapshotToGuest(
     snapshot, localLaunchNumber, extraFiles = false) {
+  const focusIndex = entryIndex;
+
   // On first launch, files are opened to determine navigation candidates. Don't
-  // reopen in that case. Otherwise, attempt to reopen here. Some files may be
-  // assigned null, e.g., if they have been moved to a different folder.
-  await Promise.all(snapshot.map(refreshFile));
+  // reopen in that case. Otherwise, attempt to reopen the focus file only. In
+  // future we might also open "nearby" files for preloading. However, reopening
+  // *all* files on every navigation attempt to verify they can still be
+  // navigated to adds noticeable lag in large directories.
+  if (focusIndex >= 0 && focusIndex < snapshot.length) {
+    await refreshFile(snapshot[focusIndex]);
+  } else if (snapshot.length !== 0) {
+    await refreshFile(snapshot[0]);
+  }
   if (localLaunchNumber !== globalLaunchNumber) {
     return;
   }
 
   /** @type {!LoadFilesMessage} */
   const loadFilesMessage = {
-    writableFileIndex: entryIndex,
+    writableFileIndex: focusIndex,
     // Handle can't be passed through a message pipe.
     files: snapshot.map(fd => ({
                           token: fd.token,

chromeos/components/media_app_ui/test/media_app_ui_browsertest.js

@@ -372,8 +372,11 @@ TEST_F('MediaAppUIBrowserTest', 'NavigateWithUnopenableSibling', async () => {
   assertEquals(currentFiles.length, 3);
 
   // The error stays on the third, now unopenable. But, since we've advanced,
-  // it has now rotated into the second slot.
-  assertEquals(await getFileErrors(), ',NotAllowedError,');
+  // it has now rotated into the second slot. But! Also we don't validate it
+  // until it rotates into the first slot, so the error won't be present yet.
+  // If we implement pre-loading, this expectation can change to
+  // ',NotAllowedError,'.
+  assertEquals(await getFileErrors(), ',,');
 
   // Navigate to the unopenable file and expect a graceful error.
   await advance(1);
@@ -382,7 +385,8 @@ TEST_F('MediaAppUIBrowserTest', 'NavigateWithUnopenableSibling', async () => {
   assertEquals(currentFiles.length, 3);
   assertEquals(await getFileErrors(), 'NotAllowedError,,');
 
-  // Navigating back to an openable file should still work.
+  // Navigating back to an openable file should still work, and the error should
+  // "stick".
   await advance(1);
   result = await waitForImageAndGetWidth('1.png');
   assertEquals(result, '111');