[css-grid-3][masonry] Negative margins should affect gaps between items #13574
Description
I'd like to propose a change to the spec where negative margins "eat" into the grid-gap placed between items.
The spec states that negative margins should affect the contribution of an item to the running position down to 0px (https://www.w3.org/TR/css-grid-3/#grid-lanes-layout-algorithm), but specifies that the stacking axis gap should be added to the running position of a track after an item is placed.
The running position contribution of an item in the spec right now is:
max_pos + clampToZero([outer size](https://www.w3.org/TR/css-sizing-3/#outer-size)) + [grid-gap](https://www.w3.org/TR/css-align-3/#propdef-grid-gap).
I think the formula should be changed to:
max_pos + clampToZero(outer_size + grid-gap)
If an item 1 has height: 30px, margin-top: -40px and the gap is 10px, following the spec formula, we'd get:
Whereas if we don't always add the stacking axis gap afterwards, we would end up with:
which doesn't have that awkward gap at the top of track 1.
If we were to follow the spec, a user could also continuously place items with a negative margin greater than its size, and the grid-gap would just continuously add to the running position of that track.