The problem is related to border-radius. Without the border radius on the left-side of element 1 and the right-side of element 2 there is no problem.
I found out that Internet Explorer (IE11 and Edge) show less than 1px white space between the 2 elements sometimes. So they also have rendering issues here.

A quick work-around for the IE problem is:
- Set a margin-left: -1px on the 2nd element.
- Set a z-index:1 and position:relative on the 1st element.
This way the gap is gone.

Unable to reproduce the issue on chrome 66.0.3359.139 using Windows 10 with steps mentioned in comment#0. Hovered on left/right and didn't observe any artifacts near middle border line. Attaching screenshot for reference.

@REporter: Please check the screenshot and let us know if we miss anything.Please provide a screencast or test file which accurately reproduces the issue. Also please highlight where the issue is seen.  Any further information on reproducing the issue would help in further triaging.

Thanks!

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You are probably not reproducing it in the right way. I can reproduce on multiple machines.
I made you a screenshot where I created the artifact on all elements.

The only thing I do, is move my mouse left to right over the middle border between the elements until the pixel artifacts arise.
Don't click or do anyhting other than hovering.

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PS: ZOOM IN and look at the middle border to see that the border now has pixels outside of the element.

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

Waiting on Feedback from Test.

Unable to reproduce this issue on reported version 66.0.3359.139 using Windows 10 HiDPI and normal windows 10, Mac Retina and Non-Retina devices and Linux HiDPI and normal devices with steps mentioned in comment#0. Attaching screencast for reference. 

@Reporter: Please check the screencast and let us know if we miss anything. 

As issue is not reproducible from TE end removing Needs-TestConfirmation and Needs-Feedback.

Thanks!

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I don't know if it's visible in the video. Don't understand why my screencast is in such poor quality.

Better Quality video.

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edaanen@, what is your display scale factor? Does changing it make the problem go away?

I can't reproduce at 125%, but maybe I can see something at 100% (only it's so small I can't be sure). Our tester used HighDPI, so probably 200%.

Yes at a scale factor of 125% the problem is gone!
At 100% I can see it clearly. 
If it's not clear, make a screenshot and zoom-in with Photoshop or something.
Then you can clearly see the pixel artifact.

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

I'm guessing our invalidation rect is too small, somehow, maybe due to rounding the wrong way.