The spec was updated (almost 1 year ago) to define properly
the static position of the positioned grid items:
But we didn't apply this change in our implementation.
The specific wording from the spec (https://drafts.csswg.org/css-grid/#static-position):
"The static position [CSS21] of an absolutely-positioned child of a grid container
is determined as if it were the sole grid item in a grid area whose edges
coincide with the padding edges of the grid container.
However, if the grid container parent is also the generator of
the absolutely positioned element’s containing block, instead use the grid area
determined in §11.1 With a Grid Container as Containing Block."
This was already fixed in Blink:
Created attachment 278246 [details]
Comment on attachment 278246 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=278246&action=review
> + RenderLayer* childLayer = child.layer();
Local variable should be a reference, not a pointer.
auto& childLayer = *child.layer();
Created attachment 278397 [details]
Comment on attachment 278397 [details]
Clearing flags on attachment: 278397
Committed r200572: <http://trac.webkit.org/changeset/200572>
All reviewed patches have been landed. Closing bug.