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

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 05955691af573fc99f2d0102d0b46c6a62b72e2f..4064135dcbfcb9846039c506bbd8dbe70ac366a4 100644
--- a/Source/core/rendering/RenderView.cpp
+++ b/Source/core/rendering/RenderView.cpp
@@ -1235,6 +1235,69 @@ LayoutUnit RenderView::viewportPercentageMax(float percentage) const
         * percentage / 100.f;
 }
 
+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().affectedByViewportChange())
+        return true;
+
+    // FIXME: Waiting for document().hasViewportUnits().
+    // if (document().hasViewportUnits())

[ojan, 2014/01/03 02:04:26]

We generally don't commit commented out code. In this case, you can just have
the FIXME.

Until this is fixed, doesn't this mean that we'll do the wrong thing in a
document that uses viewport units?

Can you add hasViewportUnits in a patch first and then we can commit this one?
Otherwise, I think we should add back in the runtime flag. I was hoping we
wouldn't need the flag since we could get all the functionality in one patch,
but if we need to do it in multiple patches we need the flag. 

[Xianzhu, 2014/01/03 20:46:24]

I meant this patch depends on https://codereview.chromium.org/82083002/. Timloh@
submitted that change yesterday so I can remove the FIXME and uncomment the code
now :)

+    //     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();