# TASK 59 — The properties panel is a hierarchy (titles own their content)

**Status:** ✅ Done — shipped 2026-10-21.

## What it is
The creator's ruling (2026-10-21), on the Live scene → Layers → webcam → Properties:

> Mirror Rect: needs a title "Shape" above it and **Rect should be selected as a default
> if none exists** … also, the "drag to move" help text needs to be **indented under
> "Mirror Rect" to show parentage** … same with **Geometry, Border Color, Border
> Thickness**. Background → Remove: Remove should be indented under Background as should
> Blur. Blur → Strength: Strength should be indented under Blur.

One rule covers all of it: **a title owns its content, and the content is indented one
level (10px) under it.** Two levels deep when a title is itself nested (Strength under
Blur under Background).

## The panel now reads
```
Shape                      ← NEW title
  [Mirror] [Rect] [Circle] ← the shape is a CHOICE, Rect selected by default
  Drag to move · drag the corner dot to resize
Geometry                   ← title (already there), fields indented
  X [ ]  Y [ ]
  W [ ]  H [ ]
Border Color               ← promoted to a title, the field indented
  [#2c3a5c] [..]
Border Thickness           ← promoted to a title, the slider indented
  [slider] 3
Background                 ← title, its rows indented
  Remove [pill]
  Blur [pill]
    Strength [slider] 50%  ← one level deeper, under Blur
```

## The shape: a choice, not a toggle
Before: one button whose content was `ShapeButtonText` — the **next** action ("Round"
when the camera was a rectangle). The creator could not tell what the camera *was*, only
what it would become.

Now: two `SegmentToggle` options — **Rect** and **Circle** — bound to two new settable
bools on `SceneElement`:
- `IsRectShape` — true when the clip is the plain rectangle, which is what a camera is
  when no shape was ever picked (**the creator's "Rect is selected by default if none
  exists"**).
- `IsRoundShape` — true when the clip is a circle.

Each setter flips the shape through `ToggleClipShape()` (so the pre-round rect is still
stored/restored) and **always re-raises** — a refused write (the vertical Chat layout is
rect-only, TASK 57) makes the UI snap back to the truth instead of sticking.

`ShapeButtonText` is gone (dead code after this change).

### Why the mutual exclusion lives in the model
Not in a WPF `RadioButton` group: a RadioButton group unchecks its siblings by setting
`IsChecked` **in code**, which makes WPF drop the binding ("Losing the binding for radio
button after I set the property manually in code", stackoverflow 7376373) — the panel
would silently stop reflecting the model. Two bool properties with model-side setters keep
the binding alive (stackoverflow 50980687 / 57275046 describe the same trap).

## The circle option is still not offered where it must not be
The Circle option's `Visibility` binds to `CanToggleClipShape` — false in the vertical
tier's Chat layout (TASK 57) — so the option is not even shown there, and the model
refuses the write anyway.

## Tests
`ytLive.Tests/PropertiesHierarchyTests.cs` — the ONE test for this change:
- the model: Rect is selected by default → picking Circle makes the clip Round → picking
  Rect again restores it → the vertical Chat lock still refuses the circle;
- the panel: the `Shape` title owns both options, the Circle option is the one gated by
  `CanToggleClipShape`, the drag hint is indented and follows the shape row, every titled
  block's content is indented 10px, and Strength sits at 20px (under Blur).

432/432 green at ship time.