What to look for. The green float and the blue block must have the same top edge, and both must sit inside the red strip. If the float is drawn higher than the blue block — sticking up out of the red — then the 10px of margin from the two spacers above reached box->y but not cy.

same thing without the spacers

No margin to collapse here, so this pair is the control: green and blue should line up in both cases, and the two red strips should look identical.

inline container that is not the first child

Here an empty block precedes the inline container, so the container is no longer the first child and the pending margin does reach cy. If this one lines up while the first does not, that is the condition at layout.c:4929 confirmed.

empty grid with a row gap above the strip

This is the shape korrespondent.net actually has. The banner above its header is a display:grid with a gap and no content; this port maps grid to a table, and that box ends up with height:0 while its row group sits a gap-height lower. Content after it still moves down, so box->y advances — the question is whether cy, which is what floats are placed against, advances with it.

Green and blue must still share a top edge. If the green float is drawn higher here and only here, the missing offset comes from the zero-height grid, not from margin collapsing.