Bug 1647948. Relax assertion about image aspect ratio to consider that images can go into error state. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D80826

layout/generic/nsImageFrame.cpp

@@ -1765,8 +1765,21 @@ static bool OldImageHasDifferentRatio(const nsImageFrame& aFrame,
   // If we have an image, we need to have a current request.
   // Same if we had an image.
   const bool hasRequest = true;
-  MOZ_ASSERT(currentRatio == ComputeAspectRatio(&aImage, hasRequest, aFrame),
-             "aspect-ratio got out of sync during paint? How?");
+#ifdef DEBUG
+  auto currentRatioRecomputed = ComputeAspectRatio(&aImage, hasRequest, aFrame);
+  // If the image encounters an error after decoding the size (and we run
+  // UpdateIntrinsicRatio) then the image will return the empty AspectRatio and
+  // the aspect ratio we compute here will be different from what was computed
+  // and stored before the image went into error state. It would be better to
+  // check that the image has an error here but we need an imgIRequest for that,
+  // not an imgIContainer. In lieu of that we check that
+  // aImage.GetIntrinsicRatio() returns Nothing() as it does when the image is
+  // in the error state and that the recomputed ratio is the zero ratio.
+  MOZ_ASSERT(
+      (!currentRatioRecomputed && aImage.GetIntrinsicRatio() == Nothing()) ||
+          currentRatio == currentRatioRecomputed,
+      "aspect-ratio got out of sync during paint? How?");
+#endif
   auto oldRatio = ComputeAspectRatio(aPrevImage, hasRequest, aFrame);
   return oldRatio != currentRatio;
 }