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

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 7746f348bf0c895e4966c9ba283bd36ddec8831a..faf08b9a60436adc55a6d7dc6f9f3cb4b0a39812 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -3179,8 +3179,40 @@ 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();

[Julien - ping for review, 2014/03/11 01:23:37]

Wouldn't it be possible to remove FrameView::contentsResized? All FrameView
callers want to avoid the setNeedsLayout call. The other calls are in ScrollView
when we update the scrollbars so it's possible the SetNeedsLayout call is needed
in this case.

[Xianzhu, 2014/03/11 19:05:13]

Agreed. Need to carefully check all callsites of it and test, so I'd like to do
it after this change.

+
+    if (widthChanged) {
+        setNeedsLayout();
+        return;
+    }
+
+    if (RenderView* renderView = this->renderView()) {

[Julien - ping for review, 2014/03/11 01:23:37]

This would be better with an early return, especially since it seems like a
corner case (if you don't have a render tree, you don't have anything to do
anyway).

[Xianzhu, 2014/03/11 19:05:13]

Done.

+        // If needsLayout, the next layout will do all the things required on layoutSize change.
+        if (renderView->needsLayout())
+            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)