Implement top controls show/hide functionality for main thread scrolling

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index 1aa320260c4a78fca50d761c17db32e99e3a9f38..d2855688c89191655ec5aa018be4be247e1e46bc 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -2483,6 +2483,9 @@ bool EventHandler::handleGestureScrollBegin(const PlatformGestureEvent& gestureE
 
     passScrollGestureEventToWidget(gestureEvent, m_scrollGestureHandlingNode->renderer());
 
+    if (m_frame->isMainFrame())
+        m_frame->host()->topControls().scrollBegin();
+
     return true;
 }
 
@@ -2537,10 +2540,19 @@ bool EventHandler::handleGestureScrollUpdate(const PlatformGestureEvent& gesture
         }
     }
 
+    // If this is main frame, allow top controls to scroll first and update
+    // delta accordingly
+    bool consumed = false;
+    if (m_frame->isMainFrame() && shouldTopControlsConsumeScroll(delta)) {
+        FloatSize excessDelta = m_frame->host()->topControls().scrollBy(delta);
+        consumed = excessDelta != delta;
+        delta = excessDelta;

[Rick Byers, 2015/02/19 16:38:38]

should we perhaps be returning early for the case of delta becoming 0?

[majidvp, 2015/02/19 19:18:01]

Done. Originally I didn't do this thinking that we should allow main frame to
consume horizontal scroll but if both direction are zero there is no point in
continuing.

+    }
+
     // Try to scroll the frame view.
     FrameView* view = m_frame->view();
     if (!view)
-        return false;
+        return consumed;
 
     if (scrollAreaOnBothAxes(delta, *view)) {
         setFrameWasScrolledByUser();
@@ -2549,14 +2561,14 @@ bool EventHandler::handleGestureScrollUpdate(const PlatformGestureEvent& gesture
 
     // If this is the main frame and it didn't scroll, propagate up to the pinch viewport.
     if (!m_frame->settings()->pinchVirtualViewportEnabled() || !m_frame->isMainFrame())
-        return false;
+        return consumed;
 
-    if (scrollAreaOnBothAxes(delta, m_frame->page()->frameHost().pinchViewport())) {
+    if (scrollAreaOnBothAxes(delta, m_frame->host()->pinchViewport())) {
         setFrameWasScrolledByUser();
         return true;
     }
 
-    return false;
+    return consumed;
 }
 
 void EventHandler::clearGestureScrollNodes()
@@ -3908,4 +3920,17 @@ unsigned EventHandler::accessKeyModifiers()
 #endif
 }
 
+bool EventHandler::shouldTopControlsConsumeScroll(FloatSize scrollDelta) const
+{
+    // Always consume if it's in the direction to show the top controls.
+    if (scrollDelta.height() > 0)
+        return true;
+
+    if (m_frame->view()->scrollPosition().y() < m_frame->view()->maximumScrollPosition().y())

[Rick Byers, 2015/02/19 16:38:38]

I don't fully understand the intention here, can you add a comment?  Is this
just how you verify that the frame is actually scrollable?  So if I'm scrolled
all the way to the bottom and I start scrolling up, the first frame will scroll
the content, then the second frame will start hiding top controls?  That seems
weird, is it consistent with the CC logic?

[majidvp, 2015/02/19 19:18:01]

Note that this is only relevant when scrolling down the page. The logic prevents
top controls to start hiding when there is no room for main frame to scroll
(e.g., frame is not scrollable or it is already scrolled all the way to the
bottom of the document). Added a comment to explain this.

This duplicated CC logic from here:
https://code.google.com/p/chromium/codesearch#chromium/src/cc/trees/layer_tre...

[Rick Byers, 2015/02/21 02:39:10]

Duh, sorry - I had the case inverted.  Makes sense, thanks.

+        return true;
+
+    return false;
+}
+
+
 } // namespace blink