IntersectionObserver never fires on a display:contents element

An element with display:contents generates no box, so it has no layout geometry and IntersectionObserver.observe() on it silently never fires — no error, no warning, the callback simply never runs. This bites when a CSS grid uses wrapper rows set to display:contents so children participate in the parent grid, and something later observes that wrapper as a scroll sentinel. Observe a real child that generates a box instead. The same absence of geometry breaks getBoundingClientRect (returns zeros) and position:sticky on such elements. Worth checking first whenever a scroll-triggered reveal never appears and the CSS looks correct.