If you've a column with 100% but the grid has a vertical scrollbar, scrollbar's width should be reduced from the track breadth.
Example to reproduce the issue:
<div style="display: -webkit-grid; overflow: scroll; -webkit-grid: 100% / 100%; width: 200px; height: 200px;">
<div style="background: lime;">item</div>
The width of the item is "200px", when it should be "200px - scrollbar's width".
Note that the height is working as expected.
This was already reported and fixed in Blink:
Created attachment 261110 [details]
Comment on attachment 261110 [details]
Clearing flags on attachment: 261110
Committed r189702: <http://trac.webkit.org/changeset/189702>
All reviewed patches have been landed. Closing bug.