TASK 59: the properties panel is a hierarchy, and the shape is a choice

The creator's ruling on the webcam's properties: a "Shape" title above the
Mirror/Rect row; the drag hint indented under that row "to show parentage";
the same for Geometry, Border Color, Border Thickness; Remove and Blur indented
under Background; Strength indented under Blur.

One rule covers it: a title owns its content and the content is indented one
level (10px) under it - two levels where a title is itself nested (Strength
under Blur under Background). The titles are SemiBold 11px #a0a0b0 like the
ones already there.

The shape also stops being a toggle button that shows the NEXT action ("Round"
when the camera was a rectangle) and becomes a CHOICE: two segment options,
Rect | Circle, with Rect selected by default - which is what the creator asked
for ("Rect should be selected as a default if none exists"). The old
ShapeButtonText is gone.

The mutual exclusion lives in the MODEL: IsRectShape/IsRoundShape on
SceneElement, each setter flipping the shape through ToggleClipShape() (so the
pre-round rect is still stored/restored) and always re-raising, so a refused
write - the vertical Chat layout is rect-only (TASK 57) - makes the UI snap
back instead of sticking. Deliberately NOT a WPF RadioButton group: a group
unchecks its siblings by setting IsChecked in code, which makes WPF drop the
binding (stackoverflow 7376373; the same trap measured in 50980687/57275046).

The Circle option stays hidden where CanToggleClipShape is false, so the circle
is still not offered in the vertical Chat layout.

Guard: ytLive.Tests/PropertiesHierarchyTests.cs - the model half (Rect by
default, pick circle, pick rect, the locked Chat layout refuses the circle) and
the panel half (the Shape title owns both options, the circle is the gated one,
the hint is indented and follows the row, every titled block's content is at
10px, Strength at 20px). 432/432 green, 0 warnings.

Models/SceneElement.cs

@@ -120,7 +120,36 @@ public abstract class SceneElement : INotifyPropertyChanged
         set
         {
             if (Set(ref _clipShape, value))
-                Raise(nameof(ShapeButtonText));
+            {
+                Raise(nameof(IsRectShape));
+                Raise(nameof(IsRoundShape));
+            }
+        }
+    }
+
+    /// <summary>True when the element's clip is the plain rectangle — the shape a
+    /// camera has when none was ever picked (TASK 59: the properties panel shows
+    /// the shape as a CHOICE, and Rect is the selected default).</summary>
+    public bool IsRectShape
+    {
+        get => ClipShape != ClipShape.Round;
+        set
+        {
+            // Round -> rect (restores the pre-round rect, like the toggle does).
+            if (value && ClipShape == ClipShape.Round) ToggleClipShape();
+            Raise(nameof(IsRectShape));
+        }
+    }
+
+    /// <summary>True when the element's clip is a circle (TASK 59).</summary>
+    public bool IsRoundShape
+    {
+        get => ClipShape == ClipShape.Round;
+        set
+        {
+            // Rect -> round (stores the pre-round rect, like the toggle does).
+            if (value && ClipShape != ClipShape.Round) ToggleClipShape();
+            Raise(nameof(IsRoundShape));
         }
     }
 
@@ -170,7 +199,6 @@ public abstract class SceneElement : INotifyPropertyChanged
 
     public double MirrorScale => IsMirrored ? -1 : 1;
     public string MirrorButtonText => IsMirrored ? "Unmirror" : "Mirror";
-    public string ShapeButtonText => ClipShape == ClipShape.Round ? "Rect" : "Round";
 
     // Live camera frames: the shared WriteableBitmap owned by CameraManager.
     private WriteableBitmap? _videoImageSource;

Models/index.md

@@ -8,7 +8,7 @@ Plain data types. No logic beyond what a property can carry. See
 | `Scene.cs` | A scene: `Name`, `IsHidden`, `IsChatScene`, `IsEditing`, **`HasBackdrop`** (Live-only policy flag, default off — only the canonical Live scene has one, see `SceneCatalog`), `Elements` collection (images + webcam config), `WebcamConfig` accessor |
 | `SceneCatalog.cs` | The five canonical scenes (Starting/Live/BRB/Chat/Ending) — the product, by name: `All`, `IsCanonical`, `Is(name, canonical)` (case-insensitive trim), `IsChat`, `HasBackdrop(name)` (true only for Live). Drives the empty-DB seed, the "+" missing-scenes menu, and `MainViewModel.EnforceBackdropPolicy` |
 | `Source.cs` | A source: `SourceType` enum (DisplayCapture/WindowCapture/Background/Image/TextOverlay/ChatBox/Countdown/WebSource/MediaSource/**AlertBox (TASK 43 — the celebration zone, one per layout)** — **Webcam removed**), `X`/`Y`/`Width`/`Height`, `Opacity`, `IsEnabled`, `ImageSource` (static) / `VideoImageSource` (live frames), `DisplaySource` (`IsLiveCapture` → video, else static), `ClipShape` enum (Traditional/Round), `IsMirrored` + `MirrorScale`, `MirrorButtonText`/`ShapeButtonText`, asset identity, `IsImageSource` (XAML binds this, never `Type`), **backdrop surface**: `IsBackdrop` (permanent bottom layer, never removable), `CaptureKey` (persisted `monitor:<n>`/`window:<hwnd>`/`picker:<name>`), `IsLiveCapture` (`Type` is Display/WindowCapture) |
-| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`/`virtual IsZoomLayer` (TASK 52); `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload); **TASK 57:** the geometry setters (`X`/`Y`/`Width`/`Height`) are `virtual` so a derived element can CONSTRAIN every write (the webcam clamps itself into the vertical tier's band) instead of every writer re-implementing the rule; `ClipShape` + `ToggleClipShape()` are `virtual` too, so a derived element can CONSTRAIN the shape (the Chat scene's vertical camera is rect-only) |
+| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`/`virtual IsZoomLayer` (TASK 52); `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload); **TASK 57:** the geometry setters (`X`/`Y`/`Width`/`Height`) are `virtual` so a derived element can CONSTRAIN every write (the webcam clamps itself into the vertical tier's band) instead of every writer re-implementing the rule; `ClipShape` + `ToggleClipShape()` are `virtual` too, so a derived element can CONSTRAIN the shape (the Chat scene's vertical camera is rect-only); **TASK 59:** `IsRectShape`/`IsRoundShape` — the shape as a CHOICE (the properties panel shows Rect | Circle with Rect selected by default), each a settable bool whose setter flips the shape through `ToggleClipShape()` and always re-raises, so the UI snaps back when a write is refused |
 | `VerticalCameraLayout.cs` | **TASK 57 (the "vv" feature):** the vertical (9:16) tier's camera rules as pure math — `TileWidth`/`TileHeight` (270×152 master px = 480×270 in the 1080×1920 output), `Inset` (32 master px from the band's outer edge), `IsVerticalTier(QualityOption)` (portrait tier), `AreaFor(band, bottom)` (the band splits into two 607×540 areas), `MaxSizeFor(band)`, `SpotFor(band, w, h, centreX, centreY)` (the six spots — the area and column the tile's **centre** falls in, inset, never outside the band) and `DefaultSpot` (bottom-right, the corner the 16:9 default webcam uses), `FillsArea(band, width)` (the camera is wide enough to fill a whole area = "enlarged") and `SizeFor(band, requestedWidth)` (the Chat camera's TWO sizes - the small tile or one whole 607×540 area — whichever is nearer the requested width, so the resize handle has two detents) |
 | `ZoomSceneElement.cs` | **Transient zoom box (TASK 52, creator-corrected 2026-10-08):** carries the box rect AND the anchored `CaptureX`/`CaptureY` (the fixed box/2 capture window set on `ActivateZoom`; moving the box never touches it). `IsZoomActive` flips `Kind` Static→Dynamic (leaves the baked base while active); never persisted (LayoutStore save skips non-Source/non-WebcamSceneConfig rows), never shown in the layers list or the element template (`IsZoomLayer` gates both). The compositor SKIPS the active element in every layer loop and draws its 2x content LAST (layer n+1) via `ZoomMagnifier`. |
 | `Webcam.cs` | Singleton webcam identity: `Id`, `DeviceId`, `Name` (one row app-wide) |