The CSS Grid spec says that grid cells serve as the containing block for items, while the CSS Exclusions spec says that Exclusions only affect the wrapping context of the containing block. 

However, the desired behavior is likely that an exclusion positioned in one cell affects the content of adjacent cells. 

Accordingly we may want to change the grid spec to suggest that grid items are sized as if their containing block were the grid cell, but that their containing block is actually the grid for all other purposes (e.g. interacting with surrounding content.)