The viewport-relative height can resolve smaller than the grid needs on a short
window - a laptop with a docked browser, or a display in portrait - which would
have reintroduced the squashing it was meant to fix. Three floors now: the
container, the calendar root, and each day cell.
The day cell is the one that matters: six week rows cannot render shorter than
about 660px however short the window, so the grid stays readable rather than
collapsing back to strips.
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