GFS used to complete the overscroll but with browserside touchpad fling the GSE at the end of the fling completes the overscroll action. The overscroll animation happens at the end of the fling rather than the beginning.
If it is something that needs to be fixed I can either change the overscroll controller to send unconsumed ack for GSUs in inertial state, or change the controller to complete the action on a GSU with inertial state.

An update, it lags with pull to refresh on the touchpad too.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/345011cc0bbfbae7fed92327eda76baaeb5b83fe

commit 345011cc0bbfbae7fed92327eda76baaeb5b83fe
Author: Sahel Sharify <sahel@chromium.org>
Date: Fri Jan 12 19:36:42 2018

Early touchpad fling termination when overscroll mode is not none.

This cl terminates a fling when overscroll mode is not None and the ack
for a GSU generated from the fling progress arrives. The overscroll
action gets completed on GSE generated from the early fling termination.
Without this change the overscroll action would get completed on GSE
generated at the end of fling. The early fling termination saves
overscroll completion from waiting for the fling progress completion.
This change is particularly useful when the fling velocity is large.

Bug:  797855 
Test: FlingControllerTest.EarlyFlingCancelationOnFlingStop
Change-Id: Ic73bfd4225bcc4484700e339649fb2f448e37bbd
Reviewed-on: https://chromium-review.googlesource.com/855045
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529024}
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/fling_controller.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/fling_controller.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/fling_controller_unittest.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/gesture_event_queue.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/gesture_event_queue.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/input_router.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/input_router_impl.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/input_router_impl.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/legacy_input_router_impl.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/input/legacy_input_router_impl.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/render_widget_host_impl.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/render_widget_host_impl.h
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/render_widget_host_unittest.cc
[modify] https://crrev.com/345011cc0bbfbae7fed92327eda76baaeb5b83fe/content/browser/renderer_host/render_widget_host_view_aura.cc

Issue 815441 has been merged into this issue.