{{-- M3's supporting-pane canonical layout: a focus pane, and beside it what only means something
in relation to it — the comments on a document, the details of a video.
……
M3's placement table, row by row (docs/reference/m3/foundations-supplement.md § Canonical
layouts → Supporting pane): below the focus pane at a flexible width on a compact or medium
window (below 840px), and at the side of the focus pane, a fixed 360dp, from expanded (840px).
If the relationship is parent and child — a list and what it selected — use ``.
`width`, from 840px: `fixed` (the default) is 360px, and 412px from 1200px, M3's fixed pane at
large and extra-large (§ Breakpoints); `split` gives the focus pane two-thirds and the
supporting pane the other third, the overview's split. Always on the trailing side, 24px from
the focus pane, which keeps the focus order the order on screen at every width, as M3 asks of
panes side by side (docs/reference/m3/foundations.md § Layout → Scaffold, Panes); a
right-to-left document mirrors it.
`compact`, below 840px: `below` (the default) stacks the supporting pane under the focus pane;
`sheet` docks it to the bottom of the window as a bottom sheet — M3's compact advice, "a
bottom sheet is a good way to keep focus on the primary pane while still giving access to
supporting info". The sheet shows its drag handle and `label` until the handle, a button
(`aria-expanded`), opens it; open, its content scrolls within half the window, and Escape
inside it closes it and returns focus to the handle. It clears a navigation bar and the
bottom safe area. From 840px the sheet is the side pane again, open.
`main` (or the default slot) is the focus pane, `supporting` the supporting one; `label`
names the supporting pane, an `