Don't always fully repaint on viewport resize

Source/core/rendering/RenderView.cpp

Index: Source/core/rendering/RenderView.cpp
diff --git a/Source/core/rendering/RenderView.cpp b/Source/core/rendering/RenderView.cpp
index 5e97df55f1446c9221e299fa2c2c92ac3f13fd13..e98f74023f67cec313d3521b2bc909ad107c1e30 100644
--- a/Source/core/rendering/RenderView.cpp
+++ b/Source/core/rendering/RenderView.cpp
@@ -200,8 +200,17 @@ void RenderView::layout()
     // Use calcWidth/Height to get the new width/height, since this will take the full page zoom factor into account.
     bool relayoutChildren = !shouldUsePrintingLayout() && (!m_frameView || width() != viewWidth() || height() != viewHeight());
     if (relayoutChildren) {
-        // FIXME: Should not always fully repaint on resize.
-        setShouldDoFullRepaintAfterLayout(true);
+        // It's hard to predict here which of full repaint or per-descendant repaint costs less.
+        // For vertical writing mode or width change it's more likely that per-descendant repaint
+        // eventually turns out to be full repaint but with the cost to handle more layout states
+        // and discrete repaint rects, so marking full repaint here is more likely to cost less.
+        // Otherwise, per-descendant repaint is more likely to avoid unnecessary full repaints.
+        if (!style()->isHorizontalWritingMode() || width() != viewWidth()) {
+            setShouldDoFullRepaintAfterLayout(true);
+        } else if (RenderObject* backgroundRenderer = this->backgroundRenderer()) {
+            if (backgroundRenderer->style()->backgroundImageNeedsFullRepaintOnContainerHeightChange())
+                setShouldDoFullRepaintAfterLayout(true);
+        }
 
         layoutScope.setChildNeedsLayout(this);
         for (RenderObject* child = firstChild(); child; child = child->nextSibling()) {
@@ -792,12 +801,18 @@ IntRect RenderView::unscaledDocumentRect() const
 
 bool RenderView::rootBackgroundIsEntirelyFixed() const
 {
-    RenderObject* rootObject = document().documentElement() ? document().documentElement()->renderer() : 0;
-    if (!rootObject)
-        return false;
+    if (RenderObject* backgroundRenderer = this->backgroundRenderer())
+        return backgroundRenderer->hasEntirelyFixedBackground();
+    return false;
+}
 
-    RenderObject* rootRenderer = rootObject->rendererForRootBackground();
-    return rootRenderer->hasEntirelyFixedBackground();
+RenderObject* RenderView::backgroundRenderer() const
+{
+    if (Element* documentElement = document().documentElement()) {
+        if (RenderObject* rootObject = documentElement->renderer())
+            return rootObject->rendererForRootBackground();
+    }
+    return 0;
 }
 
 LayoutRect RenderView::backgroundRect(RenderBox* backgroundRenderer) const