Web source: crop to the widget's true content rect (union of visible body elements) — bounding box tight on all four sides, pinned to (0,0)

- QueryContentBoundsAsync now returns a real JS object (not JSON.stringify — ExecuteScriptAsync
  double-encodes strings, which silently killed the old crop) and measures the union of every
  visible body element's getBoundingClientRect: the widget's actual rect, top-left offset included.
- CaptureFrame crops AT that offset (x,y) instead of from (0,0), so the widget anchors at origin
  and the box is flush on right/bottom. Fill maps the frame flush under the box.

HANDOFF.md

@@ -8,23 +8,24 @@ branches pre-1.0: all work lands on `main` per work unit.
 
 ## The web-source rendering work (2026-08-28) — CURRENT MODEL: canvas-size viewport + content crop
 
-**Root cause finally isolated:** hosting the page at the ELEMENT-box size clips full-bleed widgets
-(designed for ~1920×1080) to their upper-left chunk inside a small viewport — "image truncated to
-the upper-left corner regardless of scaling." That truncation persisted across every earlier
-model because all of them rendered the page at the box (or a mix); e917c41's full 1920×1080
-render was the only one that showed full content, and its letterbox dead space was purely the
-display `Uniform` + the box aspect mismatch — not the render.
+**Root causes finally isolated:** (1) hosting the page at the element-box size clips full-bleed
+widgets to their upper-left chunk (fixed — page now renders at the master canvas 1920×1080,
+stable). (2) The content-bounds crop that was supposed to kill right/bottom dead space never ran:
+the script returned `JSON.stringify(...)` (a string), and `ExecuteScriptAsync` JSON-encodes the
+result, so `JsonDocument.Parse` hit a string node and `GetProperty` threw → bounds stayed unset →
+no crop. ALSO the intended measure was wrong: scroll extents ≠ widget rect (they include the
+page's own margins). Fixed together: the script now returns a real OBJECT (union of every visible
+`body *` `getBoundingClientRect`), giving the widget's true rect INCLUDING its top-left offset,
+and `CaptureFrame` crops AT that offset — so the widget anchors at (0,0) and the box is tight on
+all four sides. `Stretch="Fill"` maps it flush under the box. Creator confirmed render+resize are
+solid at the canvas-size change; this commit delivers the tight/0,0 bounding box.
 
-**The model (implemented, visual verification pending):**
-- `Services/WebView2Manager.cs` — page renders at the MASTER CANVAS size (1920×1080), stable,
-  never tracked: full-bleed widgets render FULL (no upper-left truncation); dragging the box never
-  reflows the page; `overflow:hidden` (html+body) + transparent-bg injection. After each nav
-  `QueryContentBoundsAsync` reads the page's real content rect
-  (`scrollWidth/scrollHeight` vs `innerWidth/innerHeight`) and `CaptureFrame` **crops to the
-  actual content bounds anchored at (0,0)**. Immediate capture on `NavigationCompleted`.
-- `MainWindow.xaml` — web `<Image>` `Stretch="Fill"`: the cropped 1920×1080 frame maps flush under
-  the box (Box == widget bounds at every size, pinned (0,0), no dead space).
-- Invariants: **render at canvas size; crop to content; Fill to box.**
+**The CURRENT MODEL:**
+- `Services/WebView2Manager.cs` — page renders at 1920×1080 (stable); `overflow:hidden` +
+  transparent-bg injection; `QueryContentBoundsAsync` measures real widget bounds (object return
+  from JS, NOT JSON.stringify); `CaptureFrame` crops at the widget's (x,y) offset → anchored 0,0.
+- `MainWindow.xaml` — web `<Image>` `Stretch="Fill"`.
+- Invariant: **render at canvas size; crop to the widget's true bounds; Fill to the box.**
 
 **Property panel / sliders (2026-08-28):** web URI ✕ clears the textbox; web ✓/✕ icon buttons
 `IsTabStop="False"` so Tab walks X→Y→W→H→URI; Slider style `IsMoveToPointEnabled="True"`