Main-thread scroll if we have fixed-pos children and overflow:hidden.

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 894e1c2f82949d54987dec697f895c6754ed3cfd..60ffc3dea30f18e3405aeff1d5e6c8007e785c45 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -946,8 +946,12 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() co
 
     if (frameView->hasSlowRepaintObjects())
         reasons |= HasSlowRepaintObjects;
-    if (frameView->isScrollable() && hasVisibleSlowRepaintViewportConstrainedObjects(frameView))
+    FrameView::ScrollingReasons scrollingReasons = frameView->scrollingReasons();
+    if ((scrollingReasons == FrameView::NonScrollableFromLayout
+        || scrollingReasons == FrameView::Scrollable)
+        && hasVisibleSlowRepaintViewportConstrainedObjects(frameView)) {

[Ian Vollick, 2014/11/05 16:40:06]

I think this could be made clearer with some well named temporaries.

const bool mayBeScrolledByInput = scrollingReasons == FrameView::Scrollable;
const bool mayBeScrolledByScript = scrollingReasons ==
FrameView::NotScrollableExplicitlyDisabled;
const bool canScroll = mayBeScrolledByInput || mayBeScrolledByScript;

if (canScroll && hasBlahBlah(...) { ... }

[awoloszyn, 2014/11/05 19:25:11]

Done.

         reasons |= HasNonLayerViewportConstrainedObjects;
+    }
 
     return reasons;
 }