Desktop outline rail (TOC)

A right-side fast-access menu that lists every heading in the active markdown file so the reader can jump straight to a section. It mirrors the role of the file-tree on the left: the sidebar is "where can I read?", the outline is "where in this doc?". Shipped in #252.

Behavior at a glance

Mode	Outline rail
View	Visible (default), full heading tree of the current preview
Split	Visible (default), live-updates as the user edits
Edit-only	Hidden — there is no preview to navigate
PDF / image export (body.is-printing)	Hidden — never lands in the exported artifact

The user can also toggle the rail on/off via the status-bar Outline button or the Cmd/Ctrl + Shift + L shortcut. The toggle state persists across launches.

Anatomy

The outline rail is built from the headings (h1 … h6) that the markdown renderer emits into the .mid-preview container. Each heading already carries a slug-based id from the existing attachHeadingAnchors() decorator (e.g. ## Foo bar → id="foo-bar"), so the outline simply links into that anchor space — no second source of truth.

Building the tree

rebuildOutline(preview) is invoked from every code path that re-paints the preview:

• renderView() — view mode
• renderSplit() — split mode initial paint
• scheduleSplitRender() — debounced re-paints while the user types
• setMode() — mode change

The function:

1. Disconnects the previous IntersectionObserver and clears the link cache.
2. Reads preview.querySelectorAll('h1, h2, h3, h4, h5, h6').
3. For each heading, creates an <a class="mid-outline-item" data-level="N"> whose textContent is the heading text (with the trailing # anchor character stripped) and whose click handler smooth-scrolls the preview to that heading.
4. Wires an IntersectionObserver rooted on the preview with rootMargin: '0px 0px -75% 0px' so the "active" highlight triggers when a heading enters the top fifth of the visible region — the common scroll-spy pattern that avoids twitching at section boundaries.

If the document has zero headings the rail falls back to a quiet empty-state line: "No headings in this document."

Toggle + persistence

• Status-bar button (#status-outline) — clicking flips outlineHidden.
• Header X button (inside the rail) — also flips outlineHidden to true.
• Keyboard — Cmd/Ctrl + Shift + L toggles from anywhere in the app.
• The current state is written to app state via mid:patch-app-state under the key outlineHidden: boolean and restored on the next launch by the existing readAppState() startup flow.

The status-bar button reflects state via data-active="true|false" so the icon dims when the rail is hidden — matching the dim-when-inactive treatment of the repo cell.

Indentation by heading level

The rail uses data-level on each .mid-outline-item plus a stair-stepped padding-left ladder that shares the existing --mid-space-3 … --mid-space-6 token scale. h1 items sit flush at the top level and ship in heavier weight; h6 items sit ~32 px in. There is no actual nested DOM (flat list with indent), which keeps keyboard navigation linear and prevents long sub-trees from ballooning the rail's vertical footprint.

Why a separate rail (not a popover)

A popover would have hidden the outline behind a click on every section change, which defeats the "fast access" promise. A docked rail is always-on and gives ambient awareness — you can see where you are and where you could jump while you read. The 220 px width is tuned for a 13" laptop: slim enough that a 760 px-max preview still has breathing room next to a 280 px-wide sidebar.

Print rule

body.is-printing .mid-outline { display: none !important; } plus the existing body.is-printing .mid-shell { grid-template-columns: 1fr } reset means the rail never appears in PDF, PNG, or DOCX exports. The is-printing class is set briefly around window.mid.exportPDF() in exportAs('pdf') and removed in the finally block — same mechanism as the rest of the chrome.

Files

• apps/electron/renderer/index.html — <aside id="outline-rail" class="mid-outline"> after <main id="root">, plus the #status-outline button in the footer.
• apps/electron/renderer/renderer.ts — AppState.outlineHidden, the outline* module-level state, rebuildOutline(), setActiveOutlineItem(), applyOutlineVisibility(), setOutlineHidden(), toggleOutline(). Hooked into renderView, renderSplit, scheduleSplitRender, setMode, and the app-state hydration block.
• apps/electron/renderer/renderer.css — --mid-outline-w, body.has-outline .mid-shell grid, .mid-outline*, .mid-outline-item[data-level=N] indents, body.is-printing .mid-outline.

Verifying

1. Open any markdown doc with several headings — the rail appears on the right and lists every h1–h6, indented by level.
2. Click any outline item — the preview smooth-scrolls to that section.
3. Scroll the preview manually — the active item highlights as the top heading changes.
4. Switch to Edit-only mode — the rail disappears (no preview to navigate).
5. Switch back to View or Split — the rail returns.
6. Press Cmd/Ctrl + Shift + L (or click the Outline status-bar cell) — the rail toggles. Restart the app — the toggle state persists.
7. Open a doc with no headings — empty-state copy appears in the rail.
8. Export → PDF — the rail is absent from the exported file. Same for PNG and DOCX exports built from the preview.

Related

• #115 — title-bar mode toggle (View / Split / Edit) that drives the visibility logic.
• #228 — heading-anchors decorator that supplies the id slugs the rail targets.
• The print-mode rules added in earlier PDF-export work (body.is-printing chrome hide) — the outline rail simply joins that list.