We assumed that any grid item with 'auto' width will be stretched to fill all the available space in its grid area. We assumed this because grid area acts as item's container.
However, Grid Layout specification states on its Alignment section that items will be stretched by default, unless either justify-self or align-self compute to a value other than stretch or margins are auto. In those cases, grid items will auto-size to fit their contents.
Created attachment 259130 [details]
Comment on attachment 259130 [details]
Clearing flags on attachment: 259130
Committed r188582: <http://trac.webkit.org/changeset/188582>
All reviewed patches have been landed. Closing bug.
I added ENABLE(CSS_GRID_LAYOUT) guards around the use of parent()->isRenderGrid() in r188823.