## Title

Tauri overlay shows left-edge white strip and same scale yields different window sizes by pack.

## Severity

P2

## Environment

- OS: Windows
- App version/build: packaged release (`sprimo-tauri.exe`)
- Evidence screenshots:
  - `issues/screenshots/issue5.png`
  - `issues/screenshots/issue5-b.png`
  - `issues/screenshots/issue5-c.png`

## Summary

Two regressions were observed in packaged Tauri runtime:

1. A visible white strip appears on the overlay left edge on dark backgrounds.
2. At the same slider scale, main overlay window size differs by sprite pack.

## Reproduction Steps

1. Run packaged `sprimo-tauri.exe`.
2. Place overlay over a dark/black background.
3. Observe left window edge.
4. Open settings and set the same scale value on `default`, `ferris`, and `demogorgon`.
5. Compare main window footprint.

## Expected Result

- No white strip or white background bleed on transparent overlay edges.
- Same scale value produces the same main overlay window size regardless of pack.

## Actual Result

- Left edge can show a white strip.
- Window size differs across packs at same scale.

## Root Cause Analysis

1. Transparency chain was incomplete in CSS:
- `body` was transparent, but `html`/`#root` were not explicitly transparent/full-size, allowing
  white background bleed at edges in transparent frameless window mode.
2. Scale-to-window mapping depended on per-pack frame size:
- window sizing used pack `frame_width/frame_height`, so identical scale values produced different
  target sizes across packs.

## Implementation Notes

1. `frontend/tauri-ui/src/main.tsx`
- introduced canonical scale basis `512x512` for window sizing semantics.
- changed `fittedWindowSize` and `effectiveScaleForWindowSize` to use canonical dimensions.
- removed pack-dependent sizing from `fitWindowForScale`; pack remains used for rendering/splitting.
2. `frontend/tauri-ui/src/styles.css`
- made `html`, `body`, and `#root` explicit full-size transparent surfaces to avoid white bleed.
3. `docs/TAURI_RUNTIME_TESTING.md`
- added explicit checks for same-scale cross-pack window-size consistency and no edge white strip.
4. `docs/RELEASE_TESTING.md`
- added packaged verification steps for white-edge bleed and cross-pack same-scale size consistency.

## Verification

### Commands Run

- [x] `npm --prefix frontend/tauri-ui run build`
- [x] `cargo check -p sprimo-tauri`
- [ ] `just build-release-tauri`
- [ ] `just package-win-tauri`
- [ ] `just smoke-win-tauri`

### Visual Checklist

- [x] Before screenshot(s): `issues/screenshots/issue5.png`
- [x] Before screenshot(s): `issues/screenshots/issue5-b.png`
- [x] Before screenshot(s): `issues/screenshots/issue5-c.png`
- [ ] After screenshot(s): `issues/screenshots/issue5-after-YYYYMMDD-HHMMSS.png`

### Result

- Status: `Fix Implemented`
- Notes: packaged runtime verification pending.

## Status History

- `2026-02-14 00:00` - reporter - `Reported` - left white strip + same-scale size inconsistency reported.
- `2026-02-14 00:00` - codex - `Triaged` - identified transparency chain and scale-basis coupling root causes.
- `2026-02-14 00:00` - codex - `Fix Implemented` - switched to canonical scale basis and full-surface transparency.