So I boiled my problem down a simplified test case and it’s still driving me bonkers — the more so because Gecko does what I expect, but Chromium and WebKit do not. In those two, when the rendering window gets short (that is, not much height), the `canvas` element overflows its grid row, despite being set to `height: 100%`. In Gecko, it always stays within its grid row, as I want it to do. Nor can I find a way to get the Gecko behavior across the board. Argh!

https://codepen.io/meyerweb/pen/eYQXERZ?editors=1100