Reland "Avoid layout/full-repaint on view height change if possible"

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 3161 matching lines @@
         setLayoutSizeInternal(frameRect().size());
 
     ScrollView::frameRectsChanged();
 }
 
 void FrameView::setLayoutSizeInternal(const IntSize& size)
 {
     if (m_layoutSize == size)
         return;
 
+    bool widthChanged = m_layoutSize.width() != size.width();
     m_layoutSize = size;
-    contentsResized();
+
+    // Update scrollbars. Not calling this->contentsResized() to avoid setNeedsLayout.
+    ScrollView::contentsResized();
+
+    RenderView* renderView = this->renderView();
+    if (!renderView)
+        return;
+
+    // If needsLayout, the next layout will do all the things required on layoutSize change.
+    if (renderView->needsLayout())
+        return;
+
+    if (widthChanged) {
+        setNeedsLayout();
+        return;
+    }
+
+    performPreLayoutTasks();
+
+    // During pre-layout tasks, viewport media queries, viewport sizes and others may cause
+    // style update and set needsLayout flag.
+    if (renderView->needsLayout())
+        return;
+
+    if (!renderView->trySimplifiedLayoutOnHeightChange()) {
+        setNeedsLayout();
+        return;
+    }
+
+    // RenderView has finished a simplified layout for the height change. We need to perform
+    // necessary post-layout tasks for the simplified layout.
+    scheduleOrPerformPostLayoutTasks();
+    if (frame().page())
+        frame().page()->chrome().client().layoutUpdated(m_frame.get());
 }
 
 void FrameView::didAddScrollbar(Scrollbar* scrollbar, ScrollbarOrientation orientation)
 {
     ScrollableArea::didAddScrollbar(scrollbar, orientation);
     if (AXObjectCache* cache = axObjectCache())
         cache->handleScrollbarUpdate(this);
 }
 
 void FrameView::willRemoveScrollbar(Scrollbar* scrollbar, ScrollbarOrientation orientation)
 {
     ScrollableArea::willRemoveScrollbar(scrollbar, orientation);
     if (AXObjectCache* cache = axObjectCache()) {
         cache->remove(scrollbar);
         cache->handleScrollbarUpdate(this);
     }
 }
 
 } // namespace WebCore