Fix image uploads being perfectly white when canvas read access is bl…

…ocked (mastodon#11499)

Fixes mastodon#11496

app/javascript/mastodon/utils/resize_image.js

@@ -67,6 +67,14 @@ const processImage = (img, { width, height, orientation, type = 'image/png' }) =
 
  context.drawImage(img, 0, 0, width, height);
 
+ // The Tor Browser and maybe other browsers may prevent reading from canvas
+ // and return an all-white image instead. Assume reading failed if the resized
+ // image is perfectly white.
+ const imageData = context.getImageData(0, 0, width, height);
+ if (imageData.every(value => value === 255)) {
+ throw 'Failed to read from canvas';
+ }
+
  canvas.toBlob(resolve, type);
 });