To figure out whether the scrollbar can affect the subgrid's grid area, I take a step back and write a test for ordinary grid containers. All the browsers agree that the scrollbars do occupy the grid container's content area if the grid area is implicit. However, if the grid area is explicit, the scrollbar doesn't occupy the content space.
So the question is: is subgrid's area implicit or explicit? From the subgrid section in the spec:
a. ... The grid lines thus shared between the subgrid and its parent form the subgrid’s explicit grid, and its track sizes are governed by the parent grid.
f. The subgrid does not have any implicit grid tracks in the subgridded dimension(s). ...
It seems to me that the subgrid's grid area is explicit and its size is defined by the parent grid container. So my proposed patch probably makes sense.
Mats, does above sound reasonable to you?