feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

.agent/phases/complete/22_background_animation/01_fix_background_animation.md

@@ -1,60 +0,0 @@
-# Task 01 — Diagnose and fix the animated background (styles.css)
-
-**Phase:** `22_background_animation` · **Source:** `TODO.md` L5 —
-*"Fix background animation not working, just blinking"*
-
-## Objective
-Find why the phase-08 animated background reads as "just blinking" and
-fix `styles.css` so the grid drift and the glow breathe are both visibly
-and smoothly alive, per the phase-08 design comments (pure CSS, zero JS).
-
-## Work
-1. **Reproduce:** `uv run uvicorn app.main:app --reload` (db up), open `/`
-   in a visible Chromium window (Playwright or the interactive browser),
-   observe ≥15s. Note exactly what moves and what doesn't.
-2. **Diagnose per the `00_phase.md` checklist** (per-layer visibility
-   toggles, `background-position` samples on `body::before`, mask
-   inspection, occlusion check against `html`/`body` rules, glow
-   opacity-swing perception). Record findings + before-screenshot in
-   `.agent/reports/22_background_animation/` and
-   `.agent/screenshots/22_background_animation/`.
-   **ASSUMPTION (to verify, not assume):** the likely culprits, in
-   order — (a) the masked grid drift is too faint/slow to perceive,
-   (b) only the glow opacity swing is visible and it reads as a blink,
-   (c) a later rule occludes the `z-index: -1` layers. Confirm which
-   one actually fires before touching CSS; the fix must match the found
-   cause.
-3. **Fix in `frontend/assets/styles.css`** (smallest change that makes
-   the design read):
-   - grid: raise line alpha and/or the mask's visible radius and/or the
-     drift speed as needed for a clearly visible, seamless drift
-     (drift delta must still equal one 44px cell for a seamless loop —
-     if the speed changes, keep `background-position` 0→44px and only
-     move the duration);
-   - glow: if the breathe reads as a blink, narrow the opacity delta
-     (e.g. 0.8↔1) and/or lengthen the period — it must read as
-     breathing, not pulsing;
-   - keep: both layers `position: fixed; inset: 0; z-index: -1;
-     pointer-events: none`; no `filter: blur`; no JS; palette/contrast
-     untouched.
-4. **After-screenshot** (same viewport, two frames a few seconds apart
-   showing motion) into the same screenshots dir.
-5. `tests/unit/test_background_animation.py` (new — repo source-pin
-   pattern): pin the **final** `styles.css` values — both
-   `@keyframes` present, `body::before` → `bg-grid-drift linear
-   infinite`, `body::after` → `bg-glow-breathe`, both layers
-   `fixed`/`z-index: -1`/`pointer-events: none`, `html` keeps
-   `background: var(--bg)`, `body` keeps `background: transparent`.
-6. Manual re-verify: the "just blinking" perception is gone — smooth
-   drift + gentle breathe, no jank, no static frame.
-
-## Testing & Quality
-- `uv run pytest tests/unit/test_background_animation.py -v` green.
-- `uv run ruff check . && uv run pyright` clean.
-
-## Completion Criteria
-- [ ] Root cause documented (with before/after screenshots) in the
-      phase report dir.
-- [ ] Both layers visibly animate as the phase-08 design describes;
-      pure CSS, zero JS, no blur.
-- [ ] Unit pins green against the final values; lint/types clean.