Avoid flickering when showing a layer on a painted background for the first time by disabling async image decoding

https://bugs.webkit.org/show_bug.cgi?id=270330
rdar://117533495

Reviewed by Simon Fraser;

If an image is decoded asynchronously for a sizeForDrawing different from the
current one, a flicker may happen.

To avoid this flicker, decode the image synchronously if it has more than one
RenderElement in the page and the last time it was decoded asynchronously.

* LayoutTests/TestExpectations:
* Source/WebCore/loader/cache/CachedImage.h:
* Source/WebCore/platform/graphics/BitmapImage.cpp:
(WebCore::BitmapImage::destroyDecodedData):
(WebCore::BitmapImage::draw):
(WebCore::BitmapImage::lastDecodingOptions const):
(WebCore::BitmapImage::lastDecodingOptionsForTesting const): Deleted.
* Source/WebCore/platform/graphics/BitmapImage.h:
* Source/WebCore/platform/graphics/ImageObserver.h:
(WebCore::ImageObserver::numberOfClients const):
* Source/WebCore/rendering/RenderBoxModelObject.cpp:
(WebCore::RenderBoxModelObject::decodingModeForImageDraw const):
* Source/WebCore/testing/Internals.cpp:
(WebCore::Internals::imageLastDecodingOptions):

Canonical link: https://commits.webkit.org/276513@main

Author: shallawa

Source/WebCore/loader/cache/CachedImage.h

@@ -147,6 +147,7 @@ class CachedImage final : public CachedResource {
         // ImageObserver API
         URL sourceUrl() const override { return !m_cachedImages.isEmpty() ? (*m_cachedImages.begin())->url() : URL(); }
         String mimeType() const override { return !m_cachedImages.isEmpty() ? (*m_cachedImages.begin())->mimeType() : emptyString(); }
+        unsigned numberOfClients() const override { return !m_cachedImages.isEmpty() ? (*m_cachedImages.begin())->numberOfClients() : 0; }
         long long expectedContentLength() const override { return !m_cachedImages.isEmpty() ? (*m_cachedImages.begin())->expectedContentLength() : 0; }
 
         void encodedDataStatusChanged(const Image&, EncodedDataStatus) final;

Source/WebCore/platform/graphics/BitmapImage.cpp

@@ -86,6 +86,7 @@ void BitmapImage::destroyDecodedData(bool destroyAll)
     } else {
         m_source->destroyDecodedData(0, frameCount());
         m_currentFrameDecodingStatus = DecodingStatus::Invalid;
+        m_lastDecodingOptions = { DecodingMode::Auto };
     }
 
     // There's no need to throw away the decoder unless we're explicitly asked
@@ -257,6 +258,7 @@ ImageDrawResult BitmapImage::draw(GraphicsContext& context, const FloatRect& des
         // it is currently being decoded. New data may have been received since the previous request was made.
         if ((!frameIsCompatible && !frameIsBeingDecoded) || m_currentFrameDecodingStatus == DecodingStatus::Invalid) {
             LOG(Images, "BitmapImage::%s - %p - url: %s [requesting large async decoding]", __FUNCTION__, this, sourceURL().string().utf8().data());
+            m_lastDecodingOptions = { options.decodingMode() };
             m_source->requestFrameAsyncDecodingAtIndex(m_currentFrame, m_currentSubsamplingLevel, sizeForDrawing);
             m_currentFrameDecodingStatus = DecodingStatus::Decoding;
         }
@@ -300,6 +302,7 @@ ImageDrawResult BitmapImage::draw(GraphicsContext& context, const FloatRect& des
             }
             return ImageDrawResult::DidRequestDecoding;
         } else {
+            m_lastDecodingOptions = { options.decodingMode() };
             image = frameImageAtIndexCacheIfNeeded(m_currentFrame, m_currentSubsamplingLevel);
             LOG(Images, "BitmapImage::%s - %p - url: %s [an image frame will be decoded synchronously]", __FUNCTION__, this, sourceURL().string().utf8().data());
         }
@@ -666,6 +669,11 @@ unsigned BitmapImage::decodeCountForTesting() const
     return m_decodeCountForTesting;
 }
 
+DecodingOptions BitmapImage::lastDecodingOptions() const
+{
+    return m_lastDecodingOptions;
+}
+
 void BitmapImage::dump(TextStream& ts) const
 {
     Image::dump(ts);

Source/WebCore/platform/graphics/BitmapImage.h

@@ -158,6 +158,7 @@ class BitmapImage final : public Image {
 
     void imageFrameAvailableAtIndex(size_t);
     void decode(Function<void()>&&);
+    WEBCORE_EXPORT DecodingOptions lastDecodingOptions() const;
 
 private:
     WEBCORE_EXPORT BitmapImage(Ref<NativeImage>&&);
@@ -258,6 +259,7 @@ class BitmapImage final : public Image {
 #endif
 
     unsigned m_decodeCountForTesting { 0 };
+    DecodingOptions m_lastDecodingOptions { DecodingMode::Auto };
 
 #if USE(APPKIT)
     mutable RetainPtr<NSImage> m_nsImage; // A cached NSImage of all the frames. Only built lazily if someone actually queries for one.

Source/WebCore/platform/graphics/DecodingOptions.h

@@ -40,7 +40,7 @@ enum class DecodingMode : uint8_t {
 
 class DecodingOptions {
 public:
-    explicit DecodingOptions(DecodingMode decodingMode = DecodingMode::Auto)
+    DecodingOptions(DecodingMode decodingMode = DecodingMode::Auto)
         : m_decodingModeOrSize(decodingMode)
     {
     }

Source/WebCore/platform/graphics/ImageObserver.h

@@ -41,6 +41,7 @@ class ImageObserver {
 public:
     virtual URL sourceUrl() const = 0;
     virtual String mimeType() const = 0;
+    virtual unsigned numberOfClients() const { return 0; }
     virtual long long expectedContentLength() const = 0;
 
     virtual void encodedDataStatusChanged(const Image&, EncodedDataStatus) { };

Source/WebCore/rendering/RenderBoxModelObject.cpp

@@ -325,8 +325,10 @@ DecodingMode RenderBoxModelObject::decodingModeForImageDraw(const Image& image,
 
         // First tile paint.
         if (paintInfo.paintBehavior.contains(PaintBehavior::TileFirstPaint)) {
-            // And the images has not been painted in this element yet.
-            if (element() && !element()->hasEverPaintedImages())
+            // No image has been painted in this element yet and it should not flicker with previous painting.
+            auto observer = bitmapImage.imageObserver();
+            bool mayOverlapOtherClients = observer && observer->numberOfClients() > 1 && bitmapImage.lastDecodingOptions().isAsynchronous();
+            if (element() && !element()->hasEverPaintedImages() && !mayOverlapOtherClients)
                 return DecodingMode::Asynchronous;
         }