Bug 1781096 - Fix decoding="sync" when the frame is created after the image attribute. r=tnikkel

The fix is the one line in
nsImageLoadingContent::MaybeForceSyncDecoding, but I added new asserts
that should prevent this from regressing in the future.

Differential Revision: https://phabricator.services.mozilla.com/D154815

browser/base/content/test/performance/browser_windowopen.js

@@ -112,18 +112,6 @@ add_task(async function() {
             ) && // in the toolbar
             inRange(r.x1, 11, 13), // very close to the left of the screen
         },
-        {
-          name: "Initial favicon appearing after first paint",
-          condition: r => {
-            let faviconRect = win.gBrowser.tabs[0].iconImage.getBoundingClientRect();
-            return (
-              r.w == faviconRect.width &&
-              r.h == faviconRect.height &&
-              r.x1 == faviconRect.x &&
-              r.y1 == faviconRect.y
-            );
-          },
-        },
         {
           // Note that the length and x values here are a bit weird because on
           // some fonts, we appear to detect the two words separately.

dom/base/nsIImageLoadingContent.idl

@@ -127,6 +127,11 @@ interface nsIImageLoadingContent : imgINotificationObserver
    */
   [noscript, infallible] readonly attribute nsIURI currentURI;
 
+  /**
+   * Gets the sync-decoding hint set by the decoding attribute.
+   */
+  [noscript, infallible] readonly attribute boolean syncDecodingHint;
+
   /**
    * loadImageWithChannel allows data from an existing channel to be
    * used as the image data for this content node.

dom/base/nsImageLoadingContent.cpp

@@ -339,6 +339,11 @@ void nsImageLoadingContent::SetLoadingEnabled(bool aLoadingEnabled) {
   }
 }
 
+nsresult nsImageLoadingContent::GetSyncDecodingHint(bool* aHint) {
+  *aHint = mSyncDecodingHint;
+  return NS_OK;
+}
+
 already_AddRefed<Promise> nsImageLoadingContent::QueueDecodeAsync(
     ErrorResult& aRv) {
   Document* doc = GetOurOwnerDoc();
@@ -520,7 +525,8 @@ void nsImageLoadingContent::SetSyncDecodingHint(bool aHint) {
 
 void nsImageLoadingContent::MaybeForceSyncDecoding(
     bool aPrepareNextRequest, nsIFrame* aFrame /* = nullptr */) {
-  nsIFrame* frame = GetOurPrimaryImageFrame();
+  // GetOurPrimaryImageFrame() might not return the frame during frame init.
+  nsIFrame* frame = aFrame ? aFrame : GetOurPrimaryImageFrame();
   if (!frame) {
     return;
   }

layout/generic/nsImageFrame.cpp

@@ -516,6 +516,7 @@ void nsImageFrame::Init(nsIContent* aContent, nsContainerFrame* aParent,
     // We have a PresContext now, so we need to notify the image content node
     // that it can register images.
     imageLoader->FrameCreated(this);
+    AssertSyncDecodingHintIsInSync();
     if (nsIDocShell* docShell = PresContext()->GetDocShell()) {
       RefPtr<BrowsingContext> bc = docShell->GetBrowsingContext();
       mIsInObjectOrEmbed = bc->IsEmbedderTypeObjectOrEmbed() &&
@@ -2052,9 +2053,24 @@ static bool OldImageHasDifferentRatio(const nsImageFrame& aFrame,
   return oldRatio != currentRatio;
 }
 
+#ifdef DEBUG
+void nsImageFrame::AssertSyncDecodingHintIsInSync() const {
+  if (!IsForElement()) {
+    return;
+  }
+  nsCOMPtr<nsIImageLoadingContent> imageLoader = do_QueryInterface(mContent);
+  MOZ_ASSERT(imageLoader);
+
+  // The opposite is not true, we might have some other heuristics which force
+  // sync-decoding of images.
+  MOZ_ASSERT_IF(imageLoader->GetSyncDecodingHint(), mForceSyncDecoding);
+}
+#endif
+
 void nsDisplayImage::Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) {
   MOZ_ASSERT(mImage);
   auto* frame = static_cast<nsImageFrame*>(mFrame);
+  frame->AssertSyncDecodingHintIsInSync();
 
   const bool oldImageIsDifferent =
       OldImageHasDifferentRatio(*frame, *mImage, mPrevImage);
@@ -2118,6 +2134,7 @@ bool nsDisplayImage::CreateWebRenderCommands(
     return false;
   }
 
+  frame->AssertSyncDecodingHintIsInSync();
   const bool oldImageIsDifferent =
       OldImageHasDifferentRatio(*frame, *mImage, mPrevImage);
 

layout/generic/nsImageFrame.h

@@ -298,6 +298,13 @@ class nsImageFrame : public nsAtomicContainerFrame, public nsIReflowCallback {
   /// Always sync decode our image when painting if @aForce is true.
   void SetForceSyncDecoding(bool aForce) { mForceSyncDecoding = aForce; }
 
+  void AssertSyncDecodingHintIsInSync() const
+#ifndef DEBUG
+      {}
+#else
+      ;
+#endif
+
   /**
    * Computes the predicted dest rect that we'll draw into, in app units, based
    * upon the provided frame content box. (The content box is what