Report accurate Overscroll on handleWheel.

Source/core/frame/RootFrameViewport.cpp

Index: Source/core/frame/RootFrameViewport.cpp
diff --git a/Source/core/frame/RootFrameViewport.cpp b/Source/core/frame/RootFrameViewport.cpp
index 3ba978fd47f6bf304fb7f05ff327a2e0d915b761..24ebc5fb145bf96fefaae18a55fde069e684c6cd 100644
--- a/Source/core/frame/RootFrameViewport.cpp
+++ b/Source/core/frame/RootFrameViewport.cpp
@@ -96,9 +96,7 @@ ScrollResult RootFrameViewport::handleWheel(const PlatformWheelEvent& event)
 {
     updateScrollAnimator();
 
-    ScrollResult viewScrollResult;
-    if (layoutViewport().isScrollable())
-        viewScrollResult = layoutViewport().handleWheel(event);
+    ScrollResult viewScrollResult = layoutViewport().handleWheel(event);

[MuVen, 2015/06/25 12:55:51]

bokan@, handleWheel should be triggered, irrespective of isScrollable() to
calculate unUsedDelta. wdyt ? Example: 

https://css-tricks.com/examples/WebKitScrollbars/ in this case as the
layoutViewport is not scrollable(reason: NotScrollableNoOverflow), due to which
unusedDelta is not calculated. 

[bokan, 2015/06/25 14:54:35]

Sounds fine to me - it's actually an improvement - as long as all the tests
pass.

[MuVen, 2015/06/25 17:05:36]

Done.

 
     // The visual viewport will only accept pixel scrolls.
     if (!event.canScroll() || event.granularity() == ScrollByPageWheelEvent)
@@ -122,7 +120,7 @@ ScrollResult RootFrameViewport::handleWheel(const PlatformWheelEvent& event)
 
     DoublePoint usedLocationDelta(visualViewport().scrollPositionDouble() - oldOffset);
     if (!viewScrollResult.didScroll() && usedLocationDelta == DoublePoint::zero())
-        return ScrollResult();
+        return ScrollResult(false, false, viewScrollResult.unusedScrollDeltaX - usedLocationDelta.x(), viewScrollResult.unusedScrollDeltaY - usedLocationDelta.y());

[bokan, 2015/06/25 14:54:35]

This still looks wrong to me, I think you have the negatives mixed up -
admittedly they're very confusing here.

Digging through the wheel handing code, it seems scrolling down on a page will
generate PlatformWheelEvents with a negative delta.
ScrollAnimator::handleWheeEvent negates the delta, calls userScroll, then
negates the result so the ScrollResult.unusedScrollDelta is the unused *event*
delta. i.e. calling layoutViewport().handleWheel with WheelEvent{0, -10}, say it
scrolls the page down 2px, viewScrollResult.unusedScrollDeltaY == -8. This is
why we negate the locationDelta above. So this should be
-viewScrollResult.unusedScrollDelta - usedLocationDelta.

Unfortunately, it looks like the ScrollResult returned by this method has
different semantics than the base class version in ScrollableArea. That version
returns the unused portion of the PlatformWheelEvent (i.e. negative amount
remaining). This method will return the amount of *applied* delta that was
unused (i.e. positive amount remaining). We should definitely fix that (I think
it makes sense to make it always return the amount of unused applied delta) but
lets do that in a separate patch. Please leave a TODO and/or file a bug.

P.S. When testing your changes, make sure you're using
--disable-threaded-scrolling, otherwise you'll be scrolling on the compositor
and not testing these changes.

[MuVen, 2015/06/25 17:05:36]

Done. filed bug @ crbug.com/504389.

 
     DoubleSize unusedLocationDelta(locationDelta - usedLocationDelta);
     bool didScrollX = viewScrollResult.didScrollX || unusedLocationDelta.width();