Bug 799335. Fix unintenional image flash on tab switch. r=joe,dholbert

This was caused by bug 792199.

There are two parts to this fix:
1. Always decode during StartDecoding even if we already have a Decoder
(hopefully this is safe) This was a bug in the original patch.

2. After calling StartDecoding() we recheck if we're complete and abort if
we're not doing a SYNC_DECODE. Before this regression we would usually be done
decoding the image because of the decoding we did when we Locked all of the
images on tab switch.

--HG--
extra : rebase_source : 9db93075aad2f45c6cc70d27ef67468b4507642c

image/src/RasterImage.cpp

@@ -2552,9 +2552,15 @@ RasterImage::RequestDecodeCore(RequestDecodeType aDecodeType)
   if (!StoringSourceData())
     return NS_OK;
 
-  // If we've already got a full decoder running, we have nothing to do
-  if (mDecoder && !mDecoder->IsSizeDecode())
+  // If we've already got a full decoder running, we'll spend a bit of time
+  // decoding because the caller want's an image soon.
+  if (mDecoder && !mDecoder->IsSizeDecode()) {
+    if (!mDecoded && !mInDecoder && mHasSourceData && aDecodeType == SOMEWHAT_SYNCHRONOUS) {
+      SAMPLE_LABEL_PRINTF("RasterImage", "DecodeABitOf", "%s", GetURIString());
+      DecodeWorker::Singleton()->DecodeABitOf(this);
+    }
     return NS_OK;
+  }
 
   // mFinishing protects against the case when we enter RequestDecode from
   // ShutdownDecoder -- in that case, we're done with the decode, we're just

layout/base/nsCSSRendering.cpp

@@ -4014,19 +4014,25 @@ nsImageRenderer::~nsImageRenderer()
 bool
 nsImageRenderer::PrepareImage()
 {
-  if (mImage->IsEmpty() || !mImage->IsComplete()) {
+  if (mImage->IsEmpty())
+    return false;
+
+  if (!mImage->IsComplete()) {
     // Make sure the image is actually decoding
     mImage->StartDecoding();
 
-    // We can not prepare the image for rendering if it is not fully loaded.
-    //
-    // Special case: If we requested a sync decode and we have an image, push
-    // on through
-    nsCOMPtr<imgIContainer> img;
-    if (!((mFlags & FLAG_SYNC_DECODE_IMAGES) &&
-          (mType == eStyleImageType_Image) &&
-          (NS_SUCCEEDED(mImage->GetImageData()->GetImage(getter_AddRefs(img))) && img)))
-      return false;
+    // check again to see if we finished
+    if (!mImage->IsComplete()) {
+      // We can not prepare the image for rendering if it is not fully loaded.
+      //
+      // Special case: If we requested a sync decode and we have an image, push
+      // on through because the Draw() will do a sync decode then
+      nsCOMPtr<imgIContainer> img;
+      if (!((mFlags & FLAG_SYNC_DECODE_IMAGES) &&
+            (mType == eStyleImageType_Image) &&
+            (NS_SUCCEEDED(mImage->GetImageData()->GetImage(getter_AddRefs(img))) && img)))
+        return false;
+    }
   }
 
   switch (mType) {