GSB uses delta_hints to calculate scrolling chain.

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/frame/VisualViewport.h"
 
 #include <memory>
 
 #include "core/dom/Document.h"
 #include "core/frame/BrowserControls.h"
@@ skipping 1824 matching lines @@
 
   VisualViewport& visual_viewport = GetFrame()->GetPage()->GetVisualViewport();
 
   // Apply some scroll and scale from the impl-side.
   WebViewImpl()->ApplyViewportDeltas(WebFloatSize(300, 200), WebFloatSize(0, 0),
                                      WebFloatSize(0, 0), 2, 0);
 
   EXPECT_SIZE_EQ(FloatSize(300, 200), visual_viewport.GetScrollOffset());
 
   // Send a scroll event on the main thread path.
+  WebGestureEvent gsb(WebInputEvent::kGestureScrollBegin,
+                      WebInputEvent::kNoModifiers,
+                      WebInputEvent::kTimeStampForTesting);
+  gsb.SetFrameScale(1);
+  gsb.source_device = kWebGestureDeviceTouchpad;
+  gsb.data.scroll_begin.delta_x_hint = -50;
+  gsb.data.scroll_begin.delta_x_hint = -60;
+  gsb.data.scroll_begin.delta_hint_units = WebGestureEvent::kPrecisePixels;
+  GetFrame()->GetEventHandler().HandleGestureEvent(gsb);
+
   WebGestureEvent gsu(WebInputEvent::kGestureScrollUpdate,
                       WebInputEvent::kNoModifiers,
                       WebInputEvent::kTimeStampForTesting);
   gsu.SetFrameScale(1);
   gsu.source_device = kWebGestureDeviceTouchpad;
   gsu.data.scroll_update.delta_x = -50;
   gsu.data.scroll_update.delta_y = -60;
   gsu.data.scroll_update.delta_units = WebGestureEvent::kPrecisePixels;
   gsu.data.scroll_update.velocity_x = 1;
   gsu.data.scroll_update.velocity_y = 1;
@@ skipping 562 matching lines @@
                                    "    margin: 0px;"
                                    "  }"
                                    "  div { height:110vh; width: 110vw; }"
                                    "</style>"
                                    "<div></div>",
                                    base_url);
 }
 
 }  // namespace
 }  // namespace blink