Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

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

Index: third_party/WebKit/Source/core/frame/VisualViewport.cpp
diff --git a/third_party/WebKit/Source/core/frame/VisualViewport.cpp b/third_party/WebKit/Source/core/frame/VisualViewport.cpp
index fd7477384c459eb4efef22a624703aca9a7bfa1b..69454fcaf3009ce66cbfc2d2cdb5bdd5fede8576 100644
--- a/third_party/WebKit/Source/core/frame/VisualViewport.cpp
+++ b/third_party/WebKit/Source/core/frame/VisualViewport.cpp
@@ -637,9 +637,13 @@ RefPtr<WebTaskRunner> VisualViewport::GetTimerTaskRunner() const {
 
 void VisualViewport::UpdateScrollOffset(const ScrollOffset& position,
                                         ScrollType scroll_type) {
-  if (DidSetScaleOrLocation(scale_, FloatPoint(position)) &&
-      scroll_type != kAnchoringScroll)
+  if (!DidSetScaleOrLocation(scale_, FloatPoint(position)))
+    return;
+  if (IsExplicitScrollType(scroll_type)) {

[pdr., 2017/04/26 00:28:32]

This will no longer call NotifyRootFrameViewport when the scroll type is
kClampingScroll. Is that intentional?

[skobes, 2017/04/26 01:28:57]

Yes, I think it was a mistake to call it for kClampingScroll. 
NotifyRootFrameViewport exists to call ScrollAnchor::Clear which should only
occur after explicit scrolls.

     NotifyRootFrameViewport();
+    if (scroll_type != kCompositorScroll && LayerForScrolling())
+      LayerForScrolling()->PlatformLayer()->ShowScrollbars();
+  }
 }
 
 GraphicsLayer* VisualViewport::LayerForContainer() const {