Properly handle viewport-flings after a bubbling scroll

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 566fb577d2ff5f489ac82846e6d20ba075f924a7..fc3ffdb95b5898ec66958f64879851cd95d8734c 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2681,10 +2681,16 @@ InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     }
 
     did_lock_scrolling_layer_ = true;
-    if (!allow_bubbling_for_current_layer) {
+
+    // When scrolls are allowed to bubble, it's important that the original
+    // scrolling layer be preserved. This ensures that, after a scroll bubbles,
+    // the user can reverse scroll directions and immediately resume scrolling
+    // the original layer that scrolled.
+    if (!should_bubble_scrolls_)
       active_tree_->SetCurrentlyScrollingLayer(layer_impl);
+
+    if (!allow_bubbling_for_current_layer)
       break;
-    }
 
     if (allow_unrestricted_bubbling_for_current_layer) {
       pending_delta -= applied_delta;