Pass the physical scroll delta through the scroll customization path.

third_party/WebKit/Source/core/frame/TopControls.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/frame/TopControls.h"
 
 #include "core/frame/FrameHost.h"
 #include "core/page/ChromeClient.h"
 #include "platform/geometry/FloatSize.h"
 #include <algorithm> // for std::min and std::max
@@ skipping 20 matching lines @@
     visitor->trace(m_frameHost);
 }
 
 void TopControls::scrollBegin()
 {
     resetBaseline();
 }
 
 FloatSize TopControls::scrollBy(FloatSize pendingDelta)
 {
-    if ((m_permittedState == WebTopControlsShown && pendingDelta.height() < 0) || (m_permittedState == WebTopControlsHidden && pendingDelta.height() > 0))
+    if ((m_permittedState == WebTopControlsShown && pendingDelta.height() > 0) || (m_permittedState == WebTopControlsHidden && pendingDelta.height() < 0))
         return pendingDelta;
 
     if (m_height == 0)
         return pendingDelta;
 
     float oldOffset = contentOffset();
     float pageScale = m_frameHost->visualViewport().scale();
 
     // Update accumulated vertical scroll and apply it to top controls
     // Compute scroll delta in viewport space by applying page scale
     m_accumulatedScrollDelta += pendingDelta.height() * pageScale;
 
-    float newContentOffset = m_baselineContentOffset + m_accumulatedScrollDelta;
+    float newContentOffset = m_baselineContentOffset - m_accumulatedScrollDelta;
 
     setShownRatio(newContentOffset / m_height);
 
     // Reset baseline when controls are fully visible
     if (m_shownRatio == 1)
         resetBaseline();
 
     // Clamp and use the expected content offset so that we don't return
     // spurrious remaining scrolls due to the imprecision of the shownRatio.
     newContentOffset = std::min(newContentOffset, m_height);
     newContentOffset = std::max(newContentOffset, 0.f);
 
-    FloatSize appliedDelta(0, (newContentOffset - oldOffset) / pageScale);
+    // We negate the different because scrolling down (positive delta) causes
+    // top controls to hide (negative offset difference).

[tdresser, 2016/03/01 19:24:09]

Would it be clearer to just take oldOffset - newContentOffset?

different -> difference

[bokan, 2016/03/01 19:40:57]

Done. Ugh, I'm a typo machine...

+    FloatSize appliedDelta(0, -(newContentOffset - oldOffset) / pageScale);
     return pendingDelta - appliedDelta;
 }
 
 void TopControls::resetBaseline()
 {
     m_accumulatedScrollDelta = 0;
     m_baselineContentOffset = contentOffset();
 }
 
 float TopControls::layoutHeight()
@@ skipping 27 matching lines @@
 {
     if (m_height == height && m_shrinkViewport == shrinkViewport)
         return;
 
     m_height = height;
     m_shrinkViewport = shrinkViewport;
     m_frameHost->chromeClient().didUpdateTopControls();
 }
 
 } // namespace blink