Unable to reproduce the issue on ubuntu 17.10 using chrome reported version #66.0.3359.117.

Attached a screen cast for reference.

Following are the steps followed to reproduce the issue.
------------
1. Opened chrome. 
2. opened a long pdf.
3. LMB click and hold on scroll bar and moved the mouse sideways away from the bar until it stops following the mouse.
4. Released LMB.
5. returned mouse to the scroll bar and observed that the PDF did not jump to that location.

reporter@ - Could you please check the attached screen cast and please let us know if anything missed from our end.

Thanks...!!

Deleted: 836256.webm 7.5 MB

	836256.webm 7.5 MB View Download

What you did was exactly what I was originally doing which resulted in the unusual behaviour.  I have re-enabled smooth scrolling and tried it again but the bug no longer occurs by just moving the mouse away from the scrollbar(same as your recording) however I am able to re-create it by making the window lose focus by right clicking on another window/desktop while the lmb is down and then releasing it.

new steps:
1. open a long pdf
2. LMB click and hold on scroll bar and move the mouse sideways away from the bar(off the window)
3.right click on another window or the desktop to lose focus
3. Release LMB
4. return your mouse to the scroll bar and the PDF will jump to that location

I have run updates since the original bug presented itself so am unsure as to why the loss of focus occurred in the first place.

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

Additional information (sorry, doesn't seem to be an edit button?)
Step #3 RMB click can also be performed on the PDF itself followed by release of LMB and subsequent LMB click on the PDF which keeps focus on the window but loses focus on the scrollbar.

We had a similar report in  issue 825139  but it seems that's been fixed in M67. Could you give this a try in Chrome Beta channel (or Dev/Canary) and see if the issue is fixed?

@Reporter: Could you please check the issue as per comment#6, Please find the below link to download latest chrome versions and let us know if the issue still persists. Adding Needs-Feedback label.
https://www.chromium.org/getting-involved/dev-channel

Thanks!

I would like to verify that the update fixed the problem however I'm having trouble building chromium:

/bin/sh: /tmp/pamac-build-fezzik/chromium-dev/chromium-dev/src/chromium-67.0.3396.10/third_party/llvm-build/Release+Asserts/bin/clang: No such file or directory
make: *** [Makefile:23: chromium-dev] Error 127

The scrolling problem does seem to be the same as linked by bokan.

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

As per comment #6 & 8, this issue is similar to  issue 825139  and the reporter has confirmed the same, duping this issue to 825139.

Thanks..