This seems to be intended behaviour, more info: https://github.com/WICG/auxclick/issues/1

Related question: what are you trying to do that makes the event.button and event.buttons properties not enough?

Using the per-revision bisect providing the bisect results,
Good build: 55.0.2875.0(Revision:421703).
Bad build: 55.0.2876.0 (Revision:421986).

You are probably looking for a change made after 421810 (known good), but no later than 421811 (first known bad).

CHANGELOG URL:

The script might not always return single CL as suspectas some perf builds might get missing due to failure.

 https://chromium.googlesource.com/chromium/src/+log/bd027238e7f3d3c8096c1a62e9063d8cbf5d8a80..902a3d6303dfbfd49a24173777afcb4b70234a17

From the CL above, assigning the issue to the concern owner 

@mustaq- Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

Review-Url: https://codereview.chromium.org/2375493005

Note : Issue is seen only in windows and is a regression issue broken in M55 and also reproduced in latest stable , beta , dev and canary channels.

Thanks!

We are doing the same thing for "pointerup" as we are doing for "mouseup". IIRC the latter is platform-specific so somewhat mimic the native apps, but I could be wrong.

Let's investigate on the "mouseup" behavior in  Issue 149091 .