Bug 2036986 - Fix ImageDecoder failing to decode animated GIFs with zero first-frame delay. r=aosmond

Some animated GIFs have a first-frame delay of zero (or no Graphic Control Extension for the first frame). The metadata decoder terminates after parsing the first frame's image descriptor without ever encountering a second frame, so it reports the image as non-animated. Previously, `AnonymousDecoderImpl` would trust this result, set `frameCount=1`/`frameCountComplete=true`, and destroy the frame count task. This caused `ImageDecoder` to return zero for frame count making code running it consider the GIF to be non-animated, differing from the rendering result when such a GIF presents in DOM.

The fix always runs the frame count task after metadata completes, even when the metadata decoder reported non-animated. The frame count task does a full parse (with `COUNT_FRAMES` flag) and will discover additional frames. If it finds more than one frame, we update the animated flag before resolving the metadata promise to `ImageDecoder`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Differential Revision: https://phabricator.services.mozilla.com/D298735

image/ImageUtils.cpp

@@ -346,12 +346,7 @@ class AnonymousDecoderImpl final : public AnonymousDecoder {
       }
     }
 
-    if (!mMetadataResult.mAnimated) {
-      mMetadataResult.mFrameCount = 1;
-      mMetadataResult.mFrameCountComplete = true;
-      mMetadataTask = nullptr;
-      mFrameCountTask = nullptr;
-    } else if (mFrameCountTask && !mFrameCountTaskRunning) {
+    if (mFrameCountTask && !mFrameCountTaskRunning) {
       MOZ_LOG(
           sLog, LogLevel::Debug,
           ("[%p] AnonymousDecoderImpl::OnMetadata -- start frame count task",
@@ -391,6 +386,17 @@ class AnonymousDecoderImpl final : public AnonymousDecoder {
       resolve = true;
     }
 
+    // If the frame count task discovered more than one frame, the image is
+    // actually animated even if the metadata decoder missed it (e.g. a GIF
+    // whose first frame has delay_time=0).
+    if (mFrameCount > 1 && !mMetadataResult.mAnimated) {
+      MOZ_LOG(sLog, LogLevel::Debug,
+              ("[%p] AnonymousDecoderImpl::OnFrameCount -- discovered "
+               "animation, frameCount %u",
+               this, mFrameCount));
+      mMetadataResult.mAnimated = true;
+    }
+
     // If metadata completing is waiting on an updated frame count, resolve it.
     mMetadataResult.mFrameCount = mFrameCount;
     mMetadataResult.mFrameCountComplete = aComplete;

testing/web-platform/tests/webcodecs/image-decoder-animated-zero-delay.https.any.js

@@ -0,0 +1,48 @@
+// META: global=window,dedicatedworker
+
+// Bug: An animated GIF whose first frame has delay_time=0 is incorrectly
+// detected as non-animated by the metadata decoder. This causes ImageDecoder
+// to report frameCount=1 and reject decode requests for subsequent frames.
+
+promise_test(async t => {
+  let support = await ImageDecoder.isTypeSupported('image/gif');
+  assert_implements_optional(support, 'Optional codec image/gif not supported.');
+
+  let response = await fetch('animated-zero-delay.gif');
+  let buffer = await response.arrayBuffer();
+  let decoder = new ImageDecoder({data: buffer, type: 'image/gif'});
+
+  await decoder.tracks.ready;
+  assert_equals(decoder.tracks.length, 1, 'Should have one track');
+
+  let track = decoder.tracks.selectedTrack;
+  assert_true(track.animated, 'Track should be detected as animated');
+  assert_equals(track.frameCount, 2, 'Should report 2 frames');
+
+  let result0 = await decoder.decode({frameIndex: 0});
+  assert_true(result0.complete, 'Frame 0 should be complete');
+  assert_equals(result0.image.codedWidth, 2);
+  assert_equals(result0.image.codedHeight, 2);
+
+  let result1 = await decoder.decode({frameIndex: 1});
+  assert_true(result1.complete, 'Frame 1 should be complete');
+  assert_equals(result1.image.codedWidth, 2);
+  assert_equals(result1.image.codedHeight, 2);
+}, 'Test animated GIF with zero first-frame delay decodes all frames');
+
+promise_test(async t => {
+  let support = await ImageDecoder.isTypeSupported('image/gif');
+  assert_implements_optional(support, 'Optional codec image/gif not supported.');
+
+  let response = await fetch('animated-zero-delay.gif');
+  let buffer = await response.arrayBuffer();
+  let decoder = new ImageDecoder({data: buffer, type: 'image/gif'});
+
+  await decoder.completed;
+
+  let track = decoder.tracks.selectedTrack;
+  assert_true(track.animated, 'Track should be animated after completed');
+  assert_equals(track.frameCount, 2, 'Should report 2 frames after completed');
+  assert_equals(track.repetitionCount, Infinity,
+      'Should report infinite repetitions (loop=0)');
+}, 'Test animated GIF with zero first-frame delay reports correct metadata');