Optimize repaint on FrameView resize

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index db19213305ef88df50aa46dc433e04cd25b2b4f9..8ec58d08914a74881c9cdea69332274fb6a15fda 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -883,7 +883,6 @@ void FrameView::layout(bool allowSubtree)
         return;
     }
 
-    bool shouldDoFullLayout = false;
     FontCachePurgePreventer fontCachePurgePreventer;
     RenderLayer* layer;
     {
@@ -909,7 +908,7 @@ void FrameView::layout(bool allowSubtree)
         ScrollbarMode vMode;
         calculateScrollbarModesForLayoutAndSetViewportRenderer(hMode, vMode);
 
-        shouldDoFullLayout = !inSubtreeLayout && (m_firstLayout || toRenderView(rootForThisLayout)->document().printing());
+        bool shouldDoFullRepaint = !inSubtreeLayout && (m_firstLayout || toRenderView(rootForThisLayout)->document().printing());
 
         if (!inSubtreeLayout) {
             // Now set our scrollbar state for the layout.
@@ -942,7 +941,15 @@ void FrameView::layout(bool allowSubtree)
             m_size = LayoutSize(layoutSize().width(), layoutSize().height());
 
             if (oldSize != m_size) {
-                shouldDoFullLayout = 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 layout states and
+                // discrete repaint rects, so marking full repaint here is more likely to cost less.
+                // For height only changes, per-descendant repaint is more likely to avoid unnecessary
+                // full repaints.
+                if (!renderView()->style()->isHorizontalWritingMode() || oldSize.width() != m_size.width())
+                    shouldDoFullRepaint = true;
+
                 if (!m_firstLayout) {
                     RenderBox* rootRenderer = document->documentElement() ? document->documentElement()->renderBox() : 0;
                     RenderBox* bodyRenderer = rootRenderer && document->body() ? document->body()->renderBox() : 0;
@@ -958,7 +965,7 @@ void FrameView::layout(bool allowSubtree)
 
         // We need to set m_doFullRepaint before triggering layout as RenderObject::checkForRepaint
         // checks the boolean to disable local repaints.
-        m_doFullRepaint |= shouldDoFullLayout;
+        m_doFullRepaint |= shouldDoFullRepaint;
 
         performLayout(rootForThisLayout, inSubtreeLayout);
 
@@ -1770,7 +1777,10 @@ void FrameView::repaintContentRectangle(const IntRect& r)
 void FrameView::contentsResized()
 {
     ScrollView::contentsResized();
-    setNeedsLayout();
+    if (RenderView* renderView = this->renderView()) {
+        // Don't directly repaint layer in setNeedsLayout. We'll handle repaint in layout().
+        renderView->setNeedsLayout(MarkContainingBlockChain, 0, DontRepaintLayer);
+    }
 }
 
 void FrameView::scrollbarExistenceDidChange()