I noticed that I managed to write "doctyp" instead of "doctype" in my example file. The bug occurs regardless (as you can see in the fiddle)

I don't see the element moving its layout, but if I remove the 'position:sticky' I do see a change in behaviour of the box shadow being drawn on top of it.

Locally, I see

55.0.2883.87 (Official Build) (64-bit) : with and without 'position:sticky" - Shadow behind 800x600 element
58.0.3010.0 (Official Build) canary (64-bit) : with 'position:sticky' - Shadow in front of 800x600 element, without 'position:sticky' - Shadow behind 800x600 element

Andreas - can you please provide some screenshots showing the difference in behaviour you're seeing to make sure we're observing the same thing?

These are taken from the JSFiddle. In the first screenshot the header has a shadow but is offset from the top left by .5rem (same amount as the shadow).

In the second screenshot there is no shadow and the header stays where it should.

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	Screen Shot 2017-02-13 at 11.41.24.png 86.0 KB View Download

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	Screen Shot 2017-02-13 at 11.41.39.png 87.1 KB View Download

I am experiencing the same issue in Chrome 56, Safari and Firefox works fine.
Here is another example: http://codepen.io/rjonsson/pen/pRmgag

I'm seeing the same issue on my retina screen but it works as expected on my non retina screen.

Tested in Chrome Canary 58.0.3020.0 and it works as expected.