GSB uses delta_hints to calculate scrolling chain.

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

 /*
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 90 matching lines @@
                                        int delta_x = 0,
                                        int delta_y = 0) {
     WebGestureEvent event(type, WebInputEvent::kNoModifiers,
                           WebInputEvent::kTimeStampForTesting);
     event.source_device = kWebGestureDeviceTouchscreen;
     event.x = 100;
     event.y = 100;
     if (type == WebInputEvent::kGestureScrollUpdate) {
       event.data.scroll_update.delta_x = delta_x;
       event.data.scroll_update.delta_y = delta_y;
+    } else if (type == WebInputEvent::kGestureScrollBegin) {
+      event.data.scroll_begin.delta_x_hint = delta_x;
+      event.data.scroll_begin.delta_y_hint = delta_y;
     }
     return WebCoalescedInputEvent(event);
   }
 
   void VerticalScroll(float delta_y) {
     GetWebView()->HandleInputEvent(
-        GenerateEvent(WebInputEvent::kGestureScrollBegin));
+        GenerateEvent(WebInputEvent::kGestureScrollBegin, 0, delta_y));
     GetWebView()->HandleInputEvent(
         GenerateEvent(WebInputEvent::kGestureScrollUpdate, 0, delta_y));
     GetWebView()->HandleInputEvent(
         GenerateEvent(WebInputEvent::kGestureScrollEnd));
   }
 
   Element* GetElementById(const WebString& id) {
     return static_cast<Element*>(
         GetWebView()->MainFrameImpl()->GetDocument().GetElementById(id));
   }
@@ skipping 941 matching lines @@
 
     // Make sure we're not adding the browser controls height to the vh units as
     // when they're locked to hidden, the ICB fills the entire viewport already.
     EXPECT_FLOAT_EQ(200.f, abs_pos->getBoundingClientRect()->height());
     EXPECT_FLOAT_EQ(200.f, fixed_pos->getBoundingClientRect()->height());
     EXPECT_EQ(400, GetFrame()->View()->ViewportSizeForViewportUnits().Height());
   }
 }
 
 }  // namespace blink