Scrollbar size is not included in the grid container's height and it should be.
Example to reproduce the issue:
<div style="display: -webkit-grid; overflow: scroll;">
<div style="background: lime;">item</div>
The grid's height should be the item's height + the scrollbar.
Note that this is working properly for the width.
This was already reported and fixed in Blink:
Created attachment 261302 [details]
Comment on attachment 261302 [details]
Clearing flags on attachment: 261302
Committed r189860: <http://trac.webkit.org/changeset/189860>
All reviewed patches have been landed. Closing bug.