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..17ef3a933f9e7c5faf03d95d5ce12e1799a78db8 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -946,8 +946,18 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() co
 
     if (frameView->hasSlowRepaintObjects())
         reasons |= HasSlowRepaintObjects;
-    if (frameView->isScrollable() && hasVisibleSlowRepaintViewportConstrainedObjects(frameView))
+    FrameView::ScrollingReasons scrollingReasons = frameView->scrollingReasons();
+    const bool mayBeScrolledByInput = (scrollingReasons == FrameView::Scrollable);
+    const bool mayBeScrolledByScript = (scrollingReasons ==

[skobes, 2014/11/05 21:05:22]

Anything scrollable by input is also scrollable by script, so this might be
clearer as:

const bool mayBeScrolledByScript = mayBeScrolledByInput || (scrollingReasons ==
    FrameView::NotScrollableExplicitlyDisabled);

Then you can use mayBeScrolledByScript in place of canScroll in the check below.

[awoloszyn, 2014/11/05 21:33:43]

Done.

+        FrameView::NotScrollableExplicitlyDisabled);
+    const bool canScroll = mayBeScrolledByInput || mayBeScrolledByScript;
+    // TODO(awoloszyn) Currently crbug.com/304810 will let certain
+    // overflow:hidden elements scroll on the fast path, so we should not let

[skobes, 2014/11/05 21:05:22]

Nit: I think this comment should say "...scroll on the compositor thread...". 
Doesn't fast path vs. slow path refer to whether we repaint?  So a main-thread
scroll can still be "fast path" if the layer is composited.

[awoloszyn, 2014/11/05 21:33:43]

Done.

+    // this move to the fast-path as an optimization, when we have slow-repaint
+    // elements.
+    if (canScroll && hasVisibleSlowRepaintViewportConstrainedObjects(frameView)) {
         reasons |= HasNonLayerViewportConstrainedObjects;
+    }
 
     return reasons;
 }