Name the bounded frame: fill:true on the shell, and the contract minor that ships it

AGENTS.md

@@ -278,16 +278,22 @@ Revisit only if the stated reason stops holding.
   it means disclosure rather than popup: the nav tree. `shell.ejs` hand-rolls the same block for the
   profile menu (its trigger composes escaped user values and its one item is a CSRF POST form) — keep
   the two in step.
-- **The document scrolls; a bounded scroll region is opt-in.** `.app` is `min-height: 100dvh`, not
-  a `100dvh` box with `overflow: hidden`: the sidebar is `position: sticky` at full height and the
-  topbar sticks too, so both stay put while the content column flows with the page. A page that wants
-  a region scrolling inside it — a board of full-height columns, a table whose header stays put —
-  gives that region a height and scrolls within it; `.table-wrap` is `overflow-x: auto` and nothing
-  more until a page does. The inverse default clipped the first long page that did not know the rule
-  (2026-09-06), silently, in every engine: nothing in a test or a console says a page is unreachable
-  below the fold. Document scrolling is also what keeps find-in-page, anchor links, keyboard paging,
-  print and reader mode working. The `visual.spec.ts` wheel test holds this; scrollIntoView would not,
-  since a script can scroll an overflow-hidden box and a reader cannot.
+- **The document scrolls; a bounded frame is `fill: true` on the shell.** `.app` is
+  `min-height: 100dvh`, not a `100dvh` box with `overflow: hidden`: the sidebar is `position: sticky`
+  at full height and the topbar sticks with it, so the nav stays reachable — on a narrow screen the
+  hamburger is the only way into it, and with no script a long page would otherwise strand the reader.
+  Three things only the document scroller gets: **keyboard paging** unconditionally (Space, PgDn and
+  End reach a bounded region only once focus is inside it, which without script needs a focusable
+  descendant), **scroll restoration** on back/forward, and find-in-page. The inverse default clipped a
+  page silently in every engine — nothing in a test or a console says content is unreachable below the
+  fold.
+  A page that is a bounded frame — a board of full-height columns, a table whose header must stay put
+  — sets **`fill: true`** on the shell and scrolls a region inside `.app-fill` instead. The height is
+  the shell's to give, not the page's to compute: a page deriving it would have to know the topbar's
+  own height, which is a reach-through, so `.table-wrap` is `overflow-x: auto` and takes its vertical
+  scroll from `.app-fill`. The `visual.spec.ts` scroll test presses **End** rather than sending a
+  wheel event (Firefox's synthetic wheel does not reach the document) and rather than
+  `scrollIntoView`, which a script can apply to an overflow-hidden box that no reader can scroll.
 - **`ICON_NAMES` (`src/ui/icons.ts`) is a host-owned registry, not a frozen plugin contract**, so it
   is deliberately not re-exported from `@plainpages/plugin-api`. The palette may narrow when the last reference
   to an id goes, and a plugin needing one gets it re-registered in the same change. Accepted cost: an