Add infrastructure for partial layouts

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 7be6a8e208b20d245e268a390d089f7f4c9e230b..7f7157724f2f5a43f9fa60be407cbefdaf51b0aa 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -151,6 +151,7 @@
 #include "core/platform/DateComponents.h"
 #include "core/platform/HistogramSupport.h"
 #include "core/platform/Language.h"
+#include "core/platform/ScrollbarTheme.h"
 #include "core/platform/Timer.h"
 #include "core/platform/chromium/TraceEvent.h"
 #include "core/platform/network/HTTPParsers.h"
@@ -1748,6 +1749,9 @@ void Document::updateLayout()
     if (frameView && renderer() && (frameView->layoutPending() || renderer()->needsLayout()))
         frameView->layout();
 
+    if (frameView)
+        frameView->resetPartialLayoutState();
+
     setNeedsFocusedElementCheck();
 }
 
@@ -1794,6 +1798,29 @@ void Document::updateLayoutIgnorePendingStylesheets()
     m_ignorePendingStylesheets = oldIgnore;
 }
 
+void Document::tryPartialUpdateLayoutIgnorePendingStylesheets(RenderObject* stopLayoutAtRenderer)

[esprehn, 2013/08/18 03:43:15]

This should take a Node, not a renderer. You need to do a style recalc just like
in updateLayoutIgnorePendingStylesheet (you probably want to refactor that
method) to make sure the renderer for Node is correct, then you can set it as
the stop renderer.

[pdr., 2013/08/20 06:19:10]

Great catch!

I refactored the common style recalc as
"recalcStyleForLayoutIgnoringPendingStylesheets()".

+{
+    if (stopLayoutAtRenderer) {
+        // Non-overlay scrollbars can cause layout during layout.
+        // FIXME: Temporarily ignore non-overlay scrollbars for test coverage.
+        if (true || ScrollbarTheme::theme()->usesOverlayScrollbars()) {

[esprehn, 2013/08/18 03:43:15]

true? You should probably delete this.

[pdr., 2013/08/20 06:19:10]

I'd like to leave this in for a bit longer since it makes performance testing
possible on Linux. Android and OSX have overlay scrollbars which don't take up
space (or CSS causes them to always take up space). Linux unfortunately has
scrollbars that appear if there's enough content which will force a second
layout, which breaks partial layout. I've updated the comment to be explicit
about why I've left it in, and I'll remove this before landing. The layouttests
will keep me honest here ;)

+            // FIXME: Text autosizing will not work with partial layout.
+            bool canPartialLayout = true;
+            RenderObject* renderer = stopLayoutAtRenderer;
+            do {
+                if (!renderer->supportsPartialLayout()) {
+                    canPartialLayout = false;
+                    break;
+                }
+            } while ((renderer = renderer->parent()));

[esprehn, 2013/08/18 03:43:15]

This is wrong, the style recalc inside updateLayoutIgnorePendingStylesheet can
change all these renderers and then your check here is not correct.

[pdr., 2013/08/20 06:19:10]

Done.

+            if (canPartialLayout)
+                view()->setStopLayoutAtRenderer(stopLayoutAtRenderer);

[esprehn, 2013/08/18 03:43:15]

You don't call any methods on it, but storing a renderer before a recalc style
is a use after free.

[pdr., 2013/08/20 06:19:10]

No use so... just "after free"? :)

Fixed.

+        }
+    }
+
+    updateLayoutIgnorePendingStylesheets();
+}
+
 PassRefPtr<RenderStyle> Document::styleForElementIgnoringPendingStylesheets(Element* element)
 {
     ASSERT_ARG(element, element->document() == this);