Some other triggers:

— select test with mouse
— focus an input field
— unfold a hidden block (as [spoiler] or [hidden] in forums).

In these cases, the page jumps to some lengths or even scrolls to the top.

related bug: https://bugs.chromium.org/p/chromium/issues/detail?id=810158

I can reproduce in the dev channel (65.0.3325.51) as well, on Windows 7.
Looks like some field trial activated some code that makes any link scroll up a bit. I reproduced it on Wikipedia, for example - any link on https://en.wikipedia.org/wiki/Negative_pulldown - even with JavaScript and relevant extensions disabled (whatever content script that page ran was disabled).
You probably have to scroll down a bit first in order to have somewhere to scroll up to when the bug occurs.

I tried to debug it before disabling JavaScript. For me, even a left click does that and it does so on mouse down, so I added an event listener breakpoint on mouse down in the Developer Tools and it never broke (and the issue occurred).

Also, it only happens once per page load. After that, the bug disappears.

And it does so all of the time, it is not sporadic at all.

Do you have experimental web platform features turned on? Could be 
root layer scrolling.

Please post the debug data for the field trials also, thanks.

It seems toggling #enable-scroll-anchoring from "Enabled" to "Default" fix the issue.

I still can't reproduce this. Can you provide a precise sequence of actions
to reproduce?

No, sorry. As I have written above, it is hard for me to catch surely reproducible algorithm.

When using Google Search, middle clicking on results (after I scrolled a bit) is impossible. Only right clicking works. Middle clicking always scrolls up and cancels the click. This is horrible.
Like I mentioned before, this is reproducible on Chrome 65.0.3325.51 dev as well, so you might want to change the M-66 label to M-65, unless you want users to pull their hair or swear. :(
(I really hope this does not happen in Chrome 64, but I cannot confirm that at the moment...)


#5 - I do not have "Experimental Web Platform features" enabled.
I only have "Developer Tools experiments" and "Experimental Extension APIs" enabled.
"Scroll Anchoring" is set to "Default".

#6 -
Variations
bd23585d-f23d1dea
c134752e-1ba87303
fe69e053-94941f92
16e0dd70-3f4a17df
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bbb8f811-3f4a17df
94e68624-3f4a17df
493ac2c5-f13e099b
da4aaa01-4d2fac87

And I believe it also happens without even clicking. :(
(It happens here as well, if I scroll a bit and then type my comment, as it centers the viewport on the cursor, even though it has not scrolled off the viewport at all)

This is very disturbing.

this is related to Scroll Anchor Serialization and a fix is being worked upon in  issue 810474 , so duping there.