Refactor root element background painting

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 1594 matching lines @@
     if (body) {
         bodyStyle = body->mutableComputedStyle();
         if (didRecalcDocumentElement)
             body->clearAnimationStyleChange();
         if (!bodyStyle || body->needsStyleRecalc() || didRecalcDocumentElement)
             bodyStyle = ensureStyleResolver().styleForElement(body, documentElementStyle.get());
         rootWritingMode = bodyStyle->writingMode();
         rootDirection = bodyStyle->direction();
     }
 
-    RefPtr<ComputedStyle> overflowStyle;
+    const ComputedStyle* backgroundStyle = documentElementStyle.get();
+    if (isHTMLHtmlElement(documentElement()) && isHTMLBodyElement(body) && !backgroundStyle->hasBackground())

[chrishtr, 2015/06/03 18:15:15]

Comment why you are doing this conditional override.

[trchen, 2015/06/03 23:15:24]

Acknowledged.

+        backgroundStyle = bodyStyle.get();
+    Color backgroundColor = backgroundStyle->visitedDependentColor(CSSPropertyBackgroundColor);
+    FillLayer backgroundLayers = backgroundStyle->backgroundLayers();
+    for (auto currentLayer = &backgroundLayers; currentLayer; currentLayer = currentLayer->next()) {
+        // http://www.w3.org/TR/css3-background/#root-background
+        // The root element background always have painting area of the whole canvas.
+        currentLayer->setClip(BorderFillBox);
+
+        // The root element doesn't scroll. It always propagates its layout overflow
+        // to the viewport. Positioning background against either box is equivalent to
+        // positioning against the scrolled box of the viewport.
+        if (currentLayer->attachment() == ScrollBackgroundAttachment)
+            currentLayer->setAttachment(LocalBackgroundAttachment);

[chrishtr, 2015/06/03 18:15:15]

This looks a little hacky. Is there a way to never set the
ScrollBackgroundAttachment in the first place?

[trchen, 2015/06/03 23:15:24]

They do behave like background-attachment:local on the LayoutView. This makes
many things work automatically, for example, --root-layer-scrolls
--enable-prefer-compositing-to-lcd-text.

IMO solving this at the invalidation & painting layer would require lots of
special rules -- being more hacky.

+    }
+    EImageRendering imageRendering = backgroundStyle->imageRendering();
+
+    const ComputedStyle* overflowStyle = nullptr;
     if (Element* element = viewportDefiningElement(documentElementStyle.get())) {
         if (element == body) {
-            overflowStyle = bodyStyle;
+            overflowStyle = bodyStyle.get();
         } else {
             ASSERT(element == documentElement());
-            overflowStyle = documentElementStyle;
+            overflowStyle = documentElementStyle.get();
         }
     }
 
     // Resolved rem units are stored in the matched properties cache so we need to make sure to
     // invalidate the cache if the documentElement needed to reattach or the font size changed
     // and then trigger a full document recalc. We also need to clear it here since the
     // call to styleForElement on the body above can cache bad values for rem units if the
     // documentElement's style was dirty. We could keep track of which elements depend on
     // rem units like we do for viewport styles, but we assume root font size changes are
     // rare and just invalidate the cache for now.
@@ skipping 16 matching lines @@
         // Column-gap is (ab)used by the current paged overflow implementation (in lack of other
         // ways to specify gaps between pages), so we have to propagate it too.
         columnGap = overflowStyle->columnGap();
     }
 
     WebScrollBlocksOn scrollBlocksOn = documentElementStyle->scrollBlocksOn();
 
     RefPtr<ComputedStyle> documentStyle = layoutView()->mutableStyle();
     if (documentStyle->writingMode() != rootWritingMode
         || documentStyle->direction() != rootDirection
+        || documentStyle->visitedDependentColor(CSSPropertyBackgroundColor) != backgroundColor
+        || documentStyle->backgroundLayers() != backgroundLayers
+        || documentStyle->imageRendering() != imageRendering
         || documentStyle->overflowX() != overflowX
         || documentStyle->overflowY() != overflowY
         || documentStyle->columnGap() != columnGap
         || documentStyle->scrollBlocksOn() != scrollBlocksOn) {
         RefPtr<ComputedStyle> newStyle = ComputedStyle::clone(*documentStyle);
         newStyle->setWritingMode(rootWritingMode);
         newStyle->setDirection(rootDirection);
-        newStyle->setColumnGap(columnGap);
+        newStyle->setBackgroundColor(backgroundColor);
+        newStyle->accessBackgroundLayers() = backgroundLayers;
+        newStyle->setImageRendering(imageRendering);
         newStyle->setOverflowX(overflowX);
         newStyle->setOverflowY(overflowY);
+        newStyle->setColumnGap(columnGap);
         newStyle->setScrollBlocksOn(scrollBlocksOn);
         layoutView()->setStyle(newStyle);
         setupFontBuilder(*newStyle);
     }
 
     if (body) {
         if (const ComputedStyle* style = body->computedStyle()) {
             if (style->direction() != rootDirection || style->writingMode() != rootWritingMode)
                 body->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::WritingModeChange));
         }
@@ skipping 4002 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif