I confirm: once end of scrollable element is reached (e.g. top or bottom of page) using autoscroll, it stops working as expected. Cursor still has the autoscroll image, but behaves like normal mouse.

This even happens without any scrolling - if you point the scroll arrow sideways on non-side scrollable window the behavior is the same.

 Issue 883152  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bdaa29eb9df18cdd6c9b9819dd0438b0eb1deef9

commit bdaa29eb9df18cdd6c9b9819dd0438b0eb1deef9
Author: Sahel Sharify <sahel@chromium.org>
Date: Mon Sep 17 17:42:25 2018

Autoscroll works after direction change on a fully scrolled element.

Without this fix autoscrolling would not work if you start a middle
click autoscroll on a page, wait for it to fully scroll and then
move the mouse to change the direction of the autoscrolling.

The fix excludes GSUs with autoscroll source from overscroll handling
logic.

Bug:  870726 
Test: AutoscrollBrowserTest.AutoscrollDirectionChangeAfterFullyScrolled
Change-Id: Ib54abf6b7e0fb1fbeab46b0bc93e9aec4d708029
Reviewed-on: https://chromium-review.googlesource.com/1162268
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591729}
[modify] https://crrev.com/bdaa29eb9df18cdd6c9b9819dd0438b0eb1deef9/content/browser/renderer_host/input/autoscroll_browsertest.cc
[modify] https://crrev.com/bdaa29eb9df18cdd6c9b9819dd0438b0eb1deef9/content/browser/renderer_host/overscroll_controller.cc

I wonder if the test has some bug. It just failed randomly in CQ. Opened https://crbug.com/887879 for it.