[docs] Document the Google Takeout Intel macOS video thumbnail scenario (#2514)

Ref:
- https://github.com/ente-io/ente/issues/2189#issuecomment-2184095189
- https://discord.com/channels/948937918347608085/1254116633237127198

docs/docs/photos/troubleshooting/thumbnails.md

@@ -7,10 +7,49 @@ description:
 
 # Missing thumbnails
 
-## Black thumbnails
+## Web
 
-Users have reported an issue with Firefox which prevents the app from generating
-thumbnails if the "block canvas fingerprinting" setting in Firefox is enabled
-(i.e. `privacy.resistFingerprinting` is set to true in `about:config`). That
-feature blocks access to the canvas, and the app needs the canvas to generate
-thumbnails.
+**Firefox** prevents the app from generating thumbnails if the "block canvas
+fingerprinting" setting in Firefox is enabled (i.e.
+**`privacy.resistFingerprinting`** is set to true in `about:config`). The app
+needs the canvas to generate thumbnails, and that Firefox feature blocks access
+to the canvas. Ideally, Firefox should be prompting for a permission, but some
+users have reported that sometime it silently blocks access, and turning off
+that setting works.
+
+Similar issues may arise if you are using an extension that blocks access to the
+canvas.
+
+## Desktop
+
+The only known case where thumbnails might be missing on desktop is when
+uploading **videos** during a Google Takeout or folder sync on **Intel macOS**
+machines. This is because the bundled ffmpeg that we use does not work with
+Rosetta. For images, we are able to fallback to other mechanisms for generating
+the thumbnails, but for videos because of their potentially huge size, the app
+doesn't try the fallback to avoid running out of memory.
+
+In such cases, you will need to use the following workaround:
+
+1.  Obtain a copy of `ffmpeg` binary that works on your Intel macOS. For
+    example, you can install it via `brew`. If you already have `ffmpeg`
+    installed you can copying it from `/usr/local/bin` (or whereever it was
+    installed).
+
+2.  Copy or symlink it to
+    `/Applications/ente.app/Contents/Resources/app.asar.unpacked/node_modules/ffmpeg-static/ffmpeg`.
+
+Even without the workaround, thumbnail generation during video uploads via the
+normal folder selection or drag and drop will work fine (since in this case we
+have access to the video's data directly without reading it from a zip and can
+thus use the fallback).
+
+## Regenerating thumbnails
+
+There is currently no functionality to regenerate thumbnails in the above cases.
+You will need to upload the affected files again.
+
+Ente skips over files that have already been uploaded, so you can drag and drop
+the original folder or zip again after removing the files without thumbnails
+(and fixing the issue on web or adding the workaround on Intel macOS), and it'll
+only upload the files that are necessary.

web/apps/photos/src/services/upload/uploadService.ts

@@ -1037,26 +1037,24 @@ const withThumbnail = async (
         } else {
             // 3. Browser based thumbnail generation for paths.
             //
-            // There are two reasons why we could get here:
+            // We can only get here when we're running in our desktop app (since
+            // only that works with non-File uploadItems), and the thumbnail
+            // generation failed. The scenarios are:
             //
-            // - We're running under Electron, but thumbnail generation is not
-            //   available. This is currently only a specific scenario for image
-            //   files on Windows.
+            // 1. We're trying to generate an image thumbnail on Windows or on
+            //    ARM64 Linux. This won't be possible since the bundled
+            //    imagemagick doesn't yet support these OS/arch combinations.
             //
-            // - We're running under the Electron, but the thumbnail generation
-            //   otherwise failed for some exception.
+            // 2. We're trying to generate a video thumbnail on Intel macOS.
+            //    This won't be possible since the bundled ffmpeg doesn't
+            //    support Rosetta.
+            //
+            // 3. Some other arbitrary exception happened.
             //
             // The fallback in this case involves reading the entire stream into
             // memory, and passing that data across the IPC boundary in a single
             // go (i.e. not in a streaming manner). This is risky for videos of
-            // unbounded sizes, plus we shouldn't even be getting here unless
-            // something went wrong.
-            //
-            // So instead of trying to cater for arbitrary exceptions, we only
-            // run this fallback to cover for the case where thumbnail
-            // generation was not available for an image file on Windows.
-            // If/when we add support of native thumbnailing on Windows too,
-            // this entire branch can be removed.
+            // unbounded sizes, so we can only apply this fallback for images.
 
             if (fileTypeInfo.fileType == FILE_TYPE.IMAGE) {
                 const data = await readEntireStream(fileStream.stream);