Brush phase 4. On release the canvas dropped its live stroke and ran `paint.stroke` over every
point again (the whole stroke re-rendered), then re-composited the stroke's bounding box, although
the canvas already showed exactly those pixels. Long strokes froze for 100–250 ms at mouse-up
(#2068).
- `Session::offer_live_stroke` hands the finished live stroke to the next command. A
`paint.stroke` / `paint.pencil` whose command, document revision, params, seed and points match
it exactly takes its pixels instead of rendering (result `"fromLive": true`). Anything else
ignores the offer and renders as before. It still runs through the normal dispatch
(authorization, journal, one history step), and a replay renders identical pixels.
- The canvas feeds the release frame's last points to the live stroke first, then remembers the
stroke's per-step damage for one revision (`CommittedStroke`): each canvas cache re-composites
only the steps it hadn't shown, usually nothing, instead of the bounding box.
- P32 (brush release on a long stroke) becomes measurable: perf_scenarios times a 600-point
200 px soft stroke on 24 MP. 136 ms → 0.83 ms on the GPU canvas, 257 ms → 0.71 ms on the CPU
canvas (RTX 5090, release). Budget 16 ms, not enforced until a baseline run.
- Scorecard: TOOL-213-10 (⌃⌥-drag resize with readout) is done; TOOL-213-7 (live strokes for every
brush-based retouch tool) is partial (Spot Healing, Mixer Brush, Background Eraser and Pattern
Stamp still aren't live).
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