Use scroll customization primitives for touch scrolling (behind REF).

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 2607f8068404cfcac097b193fdb630258c627a04..3467c2e30ec27313a45df7cba7d0abaa777d6161 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -113,6 +113,7 @@
 #include "core/page/Page.h"
 #include "core/page/PointerLockController.h"
 #include "core/page/SpatialNavigation.h"
+#include "core/page/scrolling/ScrollState.h"
 #include "core/svg/SVGDocumentExtensions.h"
 #include "core/svg/SVGElement.h"
 #include "platform/EventDispatchForbiddenScope.h"
@@ -481,6 +482,70 @@ void Element::scrollIntoViewIfNeeded(bool centerIfNeeded)
         layoutObject()->scrollRectToVisible(bounds, ScrollAlignment::alignToEdgeIfNeeded, ScrollAlignment::alignToEdgeIfNeeded);
 }
 
+void Element::distributeScroll(ScrollState& scrollState)
+{
+    ASSERT(RuntimeEnabledFeatures::scrollCustomizationEnabled());
+    if (scrollState.fullyConsumed())
+        return;
+
+    scrollState.distributeToScrollChainDescendant();
+
+    // If the scroll doesn't propagate, and we're currently scrolling
+    // an element other than this one, don't scroll.

[Rick Byers, 2015/03/11 02:22:18]

this comment says what (i.e. repeats the code below), but not why.  Why?

[tdresser, 2015/03/20 18:00:36]

Done.

+    if (!scrollState.shouldPropagate()
+        && scrollState.deltaConsumedForScrollSequence()

[Rick Byers, 2015/03/11 02:22:18]

Can you add a comment to the declaration of deltaConsumedForScrollSequence
describing exactly what it means?  It's not entirely obvious to me.

[tdresser, 2015/03/20 18:00:36]

Can you clarify what about the comment doesn't make sense?
The comment currently states 
"Whether the scroll sequence has had any delta consumed."

Is it the term "scroll sequence" that's confusing?

[Rick Byers, 2015/03/26 21:22:48]

Ah, I was looking at the method declaration - didn't think to go look at the
field for a comment.  But that's fine.  I assume 'scroll sequence' means from
the time of scroll begin, right?  That's fine - thanks.

+        && scrollState.currentNativeScrollingElement() != this) {
+        return;
+    }
+
+    const double deltaX = scrollState.deltaX();
+    const double deltaY = scrollState.deltaY();
+
+    applyScroll(scrollState);
+
+    if (deltaX != scrollState.deltaX() || deltaY != scrollState.deltaY())
+        scrollState.setCurrentNativeScrollingElement(this);
+}
+
+void Element::applyScroll(ScrollState& scrollState)
+{
+    ASSERT(RuntimeEnabledFeatures::scrollCustomizationEnabled());
+    if (scrollState.fullyConsumed())
+        return;
+
+    const double deltaX = scrollState.deltaX();
+    const double deltaY = scrollState.deltaY();
+    bool scrolled = false;
+
+    // Handle the documentElement separately, as it scrolls the FrameView.
+    if (this == document().documentElement()) {
+        FloatSize delta(deltaX, deltaY);
+        if (document().frame()->applyScrollDelta(delta, scrollState.isBeginning()))
+            scrolled = true;
+    } else {
+        if (!layoutObject())
+            return;
+        LayoutBox* curBox = layoutObject()->enclosingBox();
+        if (deltaX && curBox->scroll(ScrollLeft, ScrollByPrecisePixel, deltaX))
+            scrolled = true;
+
+        if (deltaY && curBox->scroll(ScrollUp, ScrollByPrecisePixel, deltaY))
+            scrolled = true;
+    }
+
+    if (!scrolled)
+        return;
+
+    // We need to setCurrentNativeScrollingElement in both the
+    // distributeScroll and applyScroll default implementations so
+    // that if JS overrides one of these methods, but not the
+    // other, this bookkeeping remains accurate.
+    scrollState.setCurrentNativeScrollingElement(this);
+    scrollState.consumeDeltaNative(scrollState.deltaX(), scrollState.deltaY());

[Rick Byers, 2015/03/11 02:22:18]

If curBox->scroll(ScrollLeft) returned true, but curBox->scroll(ScrollUp) return
false, do we really want to consume the deltaY?  Seems like that would break
diagonal scrolling of nested scrollers which can each scroll in only a single
axis (a good test case to add!).

Perhaps update scrolledX/scrolledY values above and pass them here?  Perhaps
also worth adding a FIXME above saying that native elements should really only
consume the amount they move, rather than always consuming the entire thing?

[tdresser, 2015/03/20 18:00:36]

Done.

+    if (scrollState.fromUserInput())
+        document().frame()->view()->setWasScrolledByUser(true);
+};
+
 static float localZoomForRenderer(LayoutObject& renderer)
 {
     // FIXME: This does the wrong thing if two opposing zooms are in effect and canceled each