Reland "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 979 matching lines @@
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewWidth(IncludeScrollbars) / scale;
 }
 
 double RenderView::layoutViewportHeight() const
 {
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewHeight(IncludeScrollbars) / scale;
 }
 
+void RenderView::viewResized()
+{
+    if (needsLayout())
+        return;
+
+    if (width() != viewWidth() || !trySimplifiedLayoutOnHeightChange())
+        setNeedsLayout();
+}
+
+bool RenderView::trySimplifiedLayoutOnHeightChange()
+{
+    // 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())

[esprehn, 2014/03/10 21:32:30]

Why not in quirks?

[Xianzhu, 2014/03/11 00:55:13]

Added comments.

+        return false;
+
+    // Optimize layout in horizontal writing mode only to simplify the logic.
+    if (!style()->isHorizontalWritingMode())
+        return false;
+
+    if (hasPercentHeightDescendants())
+        return false;
+
+    // If the height is decreasing and the view isn't vertically scrollable, fallback to
+    // full layout to calculate the new layout overflow rect.
+    LayoutRect layoutOverflow = layoutOverflowRect();
+    if (layoutOverflow.maxY() <= noOverflowRect().maxY() && viewHeight() < height())
+        return false;
+
+    if (document().ensureStyleResolver().mediaQueryAffectedByViewportChange() || document().hasViewportUnits())

[esprehn, 2014/03/10 21:32:30]

Doesn't this exclude all responsive websites?

[Xianzhu, 2014/03/11 00:55:13]

Optimized viewport media query and viewport sizes in the new Patch Set by
calling performPreLayoutTasks() from FrameView::setLayoutSizeInternal() before
calling this method. If viewport media query causes style changes, or there are
any viewport sizes, fallback to a full layout.

If media query doesn't change style, go through this path.

+        return false;
+
+    // Needs layout if there is no body element (e.g. there is frameset).
+    Element* body = document().body();
+    if (!body || !body->hasTagName(HTMLNames::bodyTag))

[esprehn, 2014/03/10 21:32:30]

Why is frameset special?

[Xianzhu, 2014/03/11 00:55:13]

Added comment.

+        return false;
+
+    // Root background image may be stretched related to the viewport size.
+    Element* documentElement = document().documentElement();
+    if (!documentElement || !documentElement->renderer()
+        || documentElement->renderer()->rendererForRootBackground()->style()->hasBackgroundImage())

[esprehn, 2014/03/10 21:32:30]

You can't do this, the style might be dirty and you didn't update it before
asking questions about it.

[Xianzhu, 2014/03/11 00:55:13]

Now update style in FrameView::setLayoutSizeInternal() (by calling
performPreLayoutTasks()) before calling this function.

+        return false;
+
+    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 false;
+
+            RenderStyle* childStyle = (*it)->style();
+            // Fixed position element may change compositing state when viewport height changes.
+            if (childStyle->position() == FixedPosition)
+                return false;
+            if (childStyle->top().isPercent() || childStyle->height().isPercent() || childStyle->minHeight().isPercent() || childStyle->maxHeight().isPercent() || !childStyle->bottom().isAuto())

[esprehn, 2014/03/10 21:32:30]

Same, you never updated style so this is all stale

[Xianzhu, 2014/03/11 00:55:13]

Ditto.

+                return false;
+        }
+    }
+
+    // Do a simplified layout on height change.
+    setHeight(viewHeight());
+
+    // During a normal layout, change of height may cause creating or destroying the overflow.
+    // The following code mimics the normal layout for overflow.
+    m_overflow.clear();
+    addLayoutOverflow(layoutOverflow);
+
+    // FIXME: There may be an more efficient way to adjust the size of the root layer.
+    layer()->updateLayerPositionsAfterLayout(layer(), RenderLayer::defaultFlags);
+
+    return true;
+}
+
 } // namespace WebCore