# TASK 52 — Zoom box (anchored 2× magnifier)

**Status:** ✅ Done — shipped 2026-10-08 (three creator corrections during the build).

## What it is
Right-clicking the live preview parks a movable **2× magnifier box** that shows a
**live, always-on magnified view of a FIXED capture region** — the box/2 window centred
on the click. The captured area is whatever was under the click; it stays the same no
matter where the box is dragged.

## Creator rulings (order received, 2026-10-08)
1. **Not a loupe.** The click fixes the capture; **moving the box never changes its
   content**. First cut shipped `4b3d82f` as a box-following loupe — reworked in
   `6bff21b` to anchored `CaptureX/Y` (drag = `UpdateZoomMove` moves box only).
2. **Content = every layer.** The box magnifies the **fully-composited frame**
   (backdrop + every element incl. webcam), drawn **LAST, on top of everything**
   (layer n+1) — `SceneCompositor` skips the active `ZoomSceneElement` in every layer
   loop and `BlitZoom`s it at the end via `Services/Compositor/ZoomMagnifier`.
3. **Never show shading.** The dim rectangles around the box must never appear inside
   it — the `DimRects` overlay is a sibling of `CanvasGrid`, never a descendant, so the
   zoom box's `VisualBrush` can't pick it up (`948dd08`).
4. **Rectangle = 4:3** ("more of a 640×480 aspect ratio"); Circle square, Phone 9:16.

## Shapes (master 1920×1080; max box 0.7·frame)
| Shape | Box | Notes |
|---|---|---|
| Circle | square, min(maxW,maxH) | round clip |
| VerticalRectangle (Phone) | 9:16 | h = 0.7·1080 |
| HorizontalRectangle (Tablet, default) | **4:3** | w = h·4/3 |

Box and capture are each centred on the click and clamped independently.

## Implementation
- `ViewModels/MainViewModel.Zoom.cs` — activate/move/dismiss + dims; transient
  `ZoomSceneElement` healed/mirrored (`HealZoomLayers`/`EnsureZoomLayer`, z-index 1, never persisted).
- `Services/Compositor/ZoomMagnifier.cs` — pure 2× bilinear over an already-composited
  opaque buffer (temp snapshot via `ArrayPool`, optional round clip).
- `Models/ZoomSceneElement.cs` / `Models/ZoomState.cs` — box rect + `CaptureX/Y`.
- `Controls/PreviewPane.xaml(.cs)` — `ZoomBox` (sibling above `CanvasGrid`, brushes the
  whole composed preview at the fixed capture, 2×) + `DimRectsLayer` (sibling above the
  content, below the box) + `UpdateZoomBox`.
- Output crop always equals the tier — the box is never the crop source.

## Tests
`ZoomMagnifierTests` (money proof: box over blue shows green capture at 2× incl. an extra
red "layer"; round-clip) + `ZoomBoxInteractionTests` (aspects, capture anchor centred on the
click, move-moves-only-the-box, corner clamp, dims-layer-structure invariant).
Full suite 406/406 on a clean run at `948dd08`.