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

Source/core/rendering/RenderView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 1204 matching lines @@
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewWidth(ScrollableArea::IncludeScrollbars) / scale;
 }
 
 double RenderView::layoutViewportHeight() const
 {
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewHeight(ScrollableArea::IncludeScrollbars) / scale;
 }
 
+void RenderView::viewResized()
+{
+    if (logicalWidth() != viewLogicalWidth() || needsLayoutOnLogicalHeightChange()) {
+        setNeedsLayout();
+    } else if (!needsLayout()) {
+        setLogicalHeight(viewLogicalHeight());
+        if (m_overflow)
+            m_overflow->setVisualOverflow(borderBoxRect());
+        layer()->updateLayerPositionsAfterLayout(layer(), RenderLayer::defaultFlags);
+    }
+}
+
+bool RenderView::needsLayoutOnLogicalHeightChange() const
+{
+    // FIXME: Move logics that are generic to RenderBox or RenderBlock into these classes
+    // so that this function can be also used to optimize layout of them.
+
+    if (document().inQuirksMode() || document().paginated() || shouldUsePrintingLayout())
+        return true;
+
+    // Optimize layout in horizontal writing mode only to simplify the logic.
+    if (!style()->isHorizontalWritingMode())
+        return true;
+
+    if (hasPercentHeightDescendants())
+        return true;
+
+    if (document().ensureStyleResolver().mediaQueryAffectedByViewportChange() || document().hasViewportUnits())
+        return true;
+
+    // Needs layout if there is no body element (e.g. there is frameset).
+    Element* body = document().body();
+    if (!body || !body->hasTagName(HTMLNames::bodyTag))
+        return true;
+
+    // Root background image may be stretched related to the viewport size.
+    Element* documentElement = document().documentElement();
+    if (!documentElement || !documentElement->renderer()
+        || documentElement->renderer()->rendererForRootBackground()->style()->hasBackgroundImage())
+        return true;
+
+    if (TrackedRendererListHashSet* positionedObjects = this->positionedObjects()) {
+        TrackedRendererListHashSet::iterator end = positionedObjects->end();
+        for (TrackedRendererListHashSet::iterator it = positionedObjects->begin(); it != end; ++it) {
+            if ((*it)->node() && (*it)->node()->hasTagName(HTMLNames::dialogTag))
+                return true;
+
+            RenderStyle* childStyle = (*it)->style();
+            // Fixed position element may change compositing state when viewport height changes.
+            if (childStyle->position() == FixedPosition)
+                return true;
+            if (childStyle->top().isPercent() || childStyle->height().isPercent() || childStyle->minHeight().isPercent() || childStyle->maxHeight().isPercent() || !childStyle->bottom().isAuto())
+                return true;
+        }
+    }
+
+    return false;
+}
+
 FragmentationDisabler::FragmentationDisabler(RenderObject* root)
 {
     RenderView* renderView = root->view();
     ASSERT(renderView);
 
     LayoutState* layoutState = renderView->layoutState();
 
     m_root = root;
     m_fragmenting = layoutState && layoutState->isPaginated();
     m_flowThreadState = m_root->flowThreadState();
@@ skipping 19 matching lines @@
 #endif
 
     if (layoutState)
         layoutState->m_isPaginated = m_fragmenting;
 
     if (m_flowThreadState != RenderObject::NotInsideFlowThread)
         m_root->setFlowThreadStateIncludingDescendants(m_flowThreadState);
 }
 
 } // namespace WebCore