Module headerline 

MG.14 — the sticky headerline every magit buffer carries.

What it answers. Each magit view is a slab of git output whose identity lives outside the text: *magit:diff* does not say which scope it diffed, *magit:blame:x.rs* does not say which revision it walked back to, the status buffer never showed the branch at all (SectionIndex::branch_status_line was written for it and never called). The headerline is the one row that answers “what am I looking at?” without re-deriving it from the body.

One provider, every view. There is exactly one [Headerline] impl here, and the per-view difference is data: a Vec<Field>, each field a string plus a FieldStyle naming its git role. No match buffer_kind, no per-kind impl — adding a view means adding a field-builder function, not a branch.

No work per tick. The cells worker calls [Headerline::version] every tick and [Headerline::render] only when it advanced. MagitHeaderline::set compares before it bumps, so a refresh that finds the same branch and the same counts costs one comparison and no repaint (paramount goal #1). Fields are produced by the SAME blocking builder that produces the buffer’s text — activation and gr alike — so the header never costs a git round-trip of its own.

Theme-live. The row resolves its colours inside render() and folds the theme’s resolved version into its own, so :colorscheme repaints the header instead of leaving it on the previous palette’s colours. The two headerlines that shipped before this one (compilation, ai-conversation) capture u32s at activation and go stale; this is the better shape and the cost is one uncontended read-lock per tick.

Design anchor: docs/dev/architecture/headerline.md, slice docs/dev/operations/slice-plans/magit.md §MG.14.

Structs

Field

One coloured run in the header row.

HeaderlineRegistration

Removes the buffer’s headerline provider when the mode deactivates.

MagitHeaderline

The one [Headerline] impl behind every magit buffer’s header row.

Enums

FieldStyle

The git role a header field plays. Maps to a theme element, which is what gives the row its identity-by-colour (the compact format carries no Head:-style labels).

Constants

MAGIT_HEADERLINE_PROVIDER_ID

Provider id tag for magit’s headerline. One per buffer scope, so a single constant covers every view — a magit buffer has exactly one major mode and therefore exactly one header.

Functions

install

Build a headerline for buffer and register it as a virtual-row provider. Returns the handle the mode keeps (to set fields as its data lands) and the registration whose drop tears the row down.

Type Aliases

MagitHeaderlineHandle

Cheap-clone handle. The mode keeps one in its per-buffer state so every refresh path can re-set the row; the registered provider holds another reference to the same allocation.