+japhet, for possible history navigation intervention (has it landed already?).

I don't know of any intervention that would cause this behavior.

Sorry, I mean a possible bug in the intervention to disallow entries without a gesture. I don't know much about the intervention so I could be way off base here though.

That intervention is currently only enabled when the user manually enables it in chrome://flags. Bugs from it would more likely present as a grayed-out back button or skipping too far, rather than not actually going back.

This would be a serious bug we would want to get fixed!  Unfortunately it's not really actionable right now, without more info on how to reproduce or what the symptoms are.  (I'm not able to repro it on Windows Canary 56.0.2901.0 with Gmail.)

groby@, can you provide more details on repro steps, or a screencast of it happening once you're in the state?  Have you seen it on any other milestones, or just M56?

I'm guessing this is Mac, based on "10.11.6"?  I'm removing the History label, because that's chrome://history and not session history (back/forward).

Steps that could help us debug the next time you run into this:
1) Open developer tools to determine whether the contents of the page are for the page you expect. This helps us determine whether this is a compositing issue
2) Hold down left-click on the back/forward buttons to determine whether the navigation state matches what you expect.
3) Describe the flickering in more details? Does the page flicker white? Or does it flicker to some other contents?

Assigning back to groby@ to gather more information.

This loosely reminds me of https://bugs.chromium.org/p/chromium/issues/detail?id=661352, which I just filed against myself to gather more debugging information. Also seeing some navigation-related issues.

Closing, since I can't repro any more either.

For posteriority: IIRC, page contents did not change, history matched what I expected. I can't recall what the flicker looked like.