Add infrastructure for partial layouts

Source/core/page/FrameView.cpp

Index: Source/core/page/FrameView.cpp
diff --git a/Source/core/page/FrameView.cpp b/Source/core/page/FrameView.cpp
index 8ed41521d20c67e53485fcd93b12ce409f7e4450..af8143660ee447e131a8ff20d27b0e3fc6b581c3 100644
--- a/Source/core/page/FrameView.cpp
+++ b/Source/core/page/FrameView.cpp
@@ -179,6 +179,7 @@ FrameView::FrameView(Frame* frame)
     , m_didRunAutosize(false)
     , m_hasSoftwareFilters(false)
     , m_visibleContentScaleFactor(1)
+    , m_partialLayout(PartialLayoutState())
 {
     init();
 
@@ -279,6 +280,7 @@ void FrameView::reset()
     m_firstVisuallyNonEmptyLayoutCallbackPending = true;
     m_maintainScrollPositionAnchor = 0;
     m_disableRepaints = 0;
+    m_partialLayout.reset();
 }
 
 void FrameView::removeFromAXObjectCache()
@@ -876,20 +878,25 @@ void FrameView::performLayout(RenderObject* rootForThisLayout, bool inSubtreeLay
     // performLayout is the actual guts of layout().
     // FIXME: The 300 other lines in layout() probably belong in other helper functions
     // so that a single human could understand what layout() is actually doing.
+    {
+        bool disableLayoutState = false;
+        if (inSubtreeLayout) {
+            RenderView* view = rootForThisLayout->view();
+            disableLayoutState = view->shouldDisableLayoutStateForSubtree(rootForThisLayout);
+            view->pushLayoutState(rootForThisLayout);
+        }
+        LayoutStateDisabler layoutStateDisabler(disableLayoutState ? rootForThisLayout->view() : 0);
 
-    bool disableLayoutState = false;
-    if (inSubtreeLayout) {
-        RenderView* view = rootForThisLayout->view();
-        disableLayoutState = view->shouldDisableLayoutStateForSubtree(rootForThisLayout);
-        view->pushLayoutState(rootForThisLayout);
-    }
-    LayoutStateDisabler layoutStateDisabler(disableLayoutState ? rootForThisLayout->view() : 0);
-
-    m_inLayout = true;
-    beginDeferredRepaints();
-    forceLayoutParentViewIfNeeded();
+        m_inLayout = true;
+        beginDeferredRepaints();
+        forceLayoutParentViewIfNeeded();
 
-    rootForThisLayout->layout(); // THIS IS WHERE LAYOUT ACTUALLY HAPPENS.
+        // Text Autosizing requires two-pass layout which is incompatible with partial layout. If
+        // enabled, only do partial layout for the second layout.
+        // FIXME (crbug.com/256657): Do not do two layouts for text autosizing.
+        PartialLayoutDisabler partialLayoutDisabler(partialLayout(), m_frame->settings() && m_frame->settings()->textAutosizingEnabled());
+        rootForThisLayout->layout(); // THIS IS WHERE LAYOUT ACTUALLY HAPPENS.
+    }
 
     bool autosized = frame()->document()->textAutosizer()->processSubtree(rootForThisLayout);
     if (autosized && rootForThisLayout->needsLayout()) {
@@ -946,6 +953,8 @@ void FrameView::layout(bool allowSubtree)
     if (m_inLayout)
         return;
 
+    ASSERT(!m_partialLayout.shouldStop());
+
     TRACE_EVENT0("webkit", "FrameView::layout");
     TRACE_EVENT_SCOPED_SAMPLING_STATE("Blink", "Layout");
 
@@ -1049,15 +1058,24 @@ void FrameView::layout(bool allowSubtree)
         layer = rootForThisLayout->enclosingLayer();
 
         m_actionScheduler->pause();
+        // Non-overlay scrollbars can cause a second layout; disable partial layout for the first of these layouts.
+        PartialLayoutDisabler partialLayoutDisabler(partialLayout(), m_frame->settings() && !m_frame->settings()->usesOverlayScrollbars());
         performLayout(rootForThisLayout, inSubtreeLayout);
         m_layoutRoot = 0;
     } // Reset m_layoutSchedulingEnabled to its previous value.
 
+    if (partialLayout().shouldStop())
+        return;
+
     bool neededFullRepaint = m_doFullRepaint;
 
     if (!inSubtreeLayout && !toRenderView(rootForThisLayout)->printing())
         adjustViewSize();
 
+    // adjustViewSize() can cause a layout due to scrollbars so a second shouldStop() check is required.
+    if (partialLayout().shouldStop())
+        return;
+
     m_doFullRepaint = neededFullRepaint;
 
     // Now update the positions of all layers.