Thanks for filing the issue!

Unable to reproduce the issue on reported chrome version 68.0.3399.0 using Windows 10 with the below mentioned steps.
1. Launched chrome and opened a web page.
2. Middle clicked mouse and tried scrolling(auto scrolled)
3. Middle clicked again and tried wheel scrolling.
We were able to scroll with out any issues. Attaching the screen cast of the same.

@Reporter: Could you please have a look at the screen cast and let us know if we have missed anything in the process. Any further inputs from your end may be helpful.

Deleted: 834219.mp4 4.9 MB

	834219.mp4 4.9 MB View Download

You shouldn't do any actions between these two middle clicks. Just middle click and middle click again, then try to scroll the wheel.

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Able to reproduce the issue on reported chrome version 68.0.3399.0 and on the latest canary 68.0.3401.0 using Windows 10.
Note: Issue is not appliocable to Mac 10.13.1 and Ubuntu 14.04

Bisect Information:
===================
Last Good Build: 67.0.3382.0
First Bad Build: 67.0.3383.0

Note: Providing Chromium bisect, as running per-revision bisect is invoking perf builds.
You are probably looking for a change made after 546619 (known good), but no later than 546637 (first known bad).
Change log:
https://chromium.googlesource.com/chromium/src/+log/8906d570277ca88252233cc404e95fcb8db635a2..0df7bcac1a864f3e62c6267807ac60ad37b35e25
Suspecting: https://chromium.googlesource.com/chromium/src/+/46259cc58e11f12ca3308b694c59a14b195fc703
Review URL: https://chromium-review.googlesource.com/980965

Note: Adding RB-Stable as this seems to be a recent regression, please remove if not required.

@sahel: Please help in assigning it to the right owner if this is not related to your change.

Thanks!