# Two-column layout: replace calc()-based heights with a viewport-locked flex shell

## Background

musehub#84 fixed the original bug (sticky sidebars clipped and couldn't scroll)
by moving from `position: sticky` to a bounded-height pattern: each column got
`max-height: calc(100dvh - var(--sticky-offset, var(--header-height)) - ...)`
plus `overflow-y: auto`.

That pattern has now broken twice, both for the same underlying reason —
`--sticky-offset` is a **static CSS guess**:

```scss
body:has(.repo-tabs) {
  --sticky-offset: calc(var(--header-height) + var(--repo-tabs-height));
}
```

It only accounts for the navbar + repo-tabs bar. Any page with additional
chrome between the header and the two-column grid (a toolbar, filter bar,
hero section, stat strip) has a wrong `--sticky-offset`, so every column's
computed `max-height` is off by however tall that extra chrome is:

1. **Footer-overlap regression** (fixed this cycle) — the calc() put on the
   *grid wrapper itself* (an earlier iteration of this pattern) claimed a
   full viewport's height measured from wherever the grid happened to sit,
   pushing the footer down to overlap still-visible content on pages with a
   hero/eyebrow above the grid.
2. **Unreachable pagination regression** (this ticket) — on the issues list
   page, `.isl-main`'s `max-height` doesn't account for the issues-list
   toolbar above `.isl-layout`, so the column's box extends past the
   visible viewport with no way to scroll to the pagination controls at the
   bottom.

Patching the formula a third time (adding yet another term to
`--sticky-offset`, or a per-page override) just sets up the next page that
adds chrome above its grid to hit the same bug again. The pattern itself —
hand-deriving "how much vertical space is left" per page in CSS — is the
defect, not any single formula.

## Goal

Replace the calc()-based height budget with a **viewport-locked flex shell**,
the same pattern Gmail/Slack/VS Code/Notion use for independently-scrollable
sibling panes: the shell claims `height: 100dvh` once, every fixed-height
sibling above the scrollable region is `flex-shrink: 0`, and the scrollable
region itself is `flex: 1; min-height: 0`. The browser computes "whatever's
left" automatically — no calc(), no magic numbers, no per-page offset to get
wrong, and this bug class becomes structurally impossible rather than
something to keep re-patching.

Confirmed with gabriel: Gmail's own layout (header fixed, no footer while
inside the app view, two panes each scroll independently within a
viewport-locked shell) is the reference UX to match.

## Design

- `base.html` gets an opt-in body class, `body-app-shell`, set only by
  templates that use a two-column layout. Pages without one (home, plain
  forms, docs/marketing pages) are untouched — normal document scroll,
  footer visible.
- `body.body-app-shell`: `height: 100dvh; overflow: hidden; display: flex;
  flex-direction: column;`. Navbar and repo-tabs: `flex-shrink: 0`.
  `#content`: `flex: 1; min-height: 0; overflow: hidden; display: flex;
  flex-direction: column;`.
- The footer is hidden on `body-app-shell` pages — matches Gmail; a
  marketing-site footer doesn't belong on a dense two-column app screen.
  Pages that keep normal scroll keep the footer exactly as today.
- Every wrapper between `#content` and the actual two-column grid (per-page
  toolbars, etc.) becomes `flex: 1; min-height: 0; display: flex;
  flex-direction: column;` so the "fill remaining space" chain isn't broken
  partway down.
- Each column (`*-main`, `*-sidebar`) keeps `min-height: 0; overflow-y: auto;`
  — no `max-height`, no `calc()`.
- `--sticky-offset` / `--repo-tabs-height` are deleted once nothing
  references them.

## Affected pages (11)

- [ ] Repo home (`_repo-home.scss` / `repo_home.html`)
- [ ] Issues list (`_issues.scss` isl-* / `issue_list.html`) — the page
      that's actually broken right now; verify this one first.
- [ ] Issue detail (`_issues.scss` isd-* / `issue_detail.html`)
- [ ] Proposal detail (`_proposal-detail.scss` / `proposal_detail.html`)
- [ ] Explore/browse (`_explore.scss` / `explore.html`)
- [ ] Docs (`_docs.scss` / `docs.html`)
- [ ] Muse dev docs (`_muse-docs.scss` / `muse_docs.html` or equivalent)
- [ ] Blame (`_blame.scss` / `blame.html`)
- [ ] Blob/outline panel (`_blob.scss` — different pattern, verify it still
      works once the shell change lands, may not need edits)
- [ ] Profile graph sidebar (`_profile.scss` `.graph-sidebar` / `profile.html`)
- [ ] `_layout.scss`'s shared `.layout-two-col`/`.layout-sidebar` utility
      classes (whichever pages actually use them, if any still do)

Each page verified individually (screenshot or live check) before moving to
the next, then merged together once all are converted and the sticky-offset
machinery is fully removed.

## Coordination note

aaronrene picked up musehub#129 (rich social preview cards / OG images) at
the same time this work started. That ticket touches
`musehub/api/routes/musehub/_ui_helpers.py`, `ui_repo.py` (backend route,
OG tag wiring), and a new `/opengraph-image` endpoint — no overlap expected
with this ticket's scope (base.html, SCSS, page templates' layout markup).
Flagging here so neither of us has to guess.