ADR-0026: Highlight warming is per-frame budgeted fill-in, not a background thread

• Status: Accepted
• Date: 2026-09-12
• Supersedes: the “warmed by a background thread” clause of ADR-0006

Context

ADR-0006 promises that syntax highlighting and intra-line emphasis run “lazily … cached per file, warmed by a background thread”. The implementation never had that thread: HighlightCache works on a per-frame line budget (FRAME_BUDGET), set at the top of every view, with unfinished work resuming across fill-in frames. The budgets the decision cares about — first paint and scrolling on giant diffs — are met by bounding work per frame, and the budget mechanics are covered by deterministic tests rather than by warming.

Decision

The background-thread warming clause is retracted. The mechanism is:

• highlight and emphasize only lines the current frame requests;
• stop at the per-frame budget and report pending work;
• resume across subsequent frames until complete.

No new thread, no shared cache invalidation, no shutdown protocol — the simplest mechanism that meets the budgets. The laziness and caching clauses of ADR-0006 stand unchanged.

Consequences

• Architecture documentation must describe budgeted fill-in, not warming.
• Performance enforcement (issue #99) rests on the deterministic budget test plus workload completion tests, not on warming behavior that was never built.