Pass the physical scroll delta through the scroll customization path.

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

Index: third_party/WebKit/Source/core/frame/TopControls.cpp
diff --git a/third_party/WebKit/Source/core/frame/TopControls.cpp b/third_party/WebKit/Source/core/frame/TopControls.cpp
index f11dbe4b4d579ce1234d42f93cc8523ccfd5251f..3c491e6236b0996ae2c421f38248e5a29aa40642 100644
--- a/third_party/WebKit/Source/core/frame/TopControls.cpp
+++ b/third_party/WebKit/Source/core/frame/TopControls.cpp
@@ -38,7 +38,7 @@ void TopControls::scrollBegin()
 
 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)
@@ -51,7 +51,7 @@ FloatSize TopControls::scrollBy(FloatSize pendingDelta)
     // 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);
 
@@ -64,7 +64,9 @@ FloatSize TopControls::scrollBy(FloatSize pendingDelta)
     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;
 }