Web source: crop to the frame's ALPHA bounding box — the widget's true rendered extent, no DOM timing, can't break the render

- FindContentBounds scans the Bgra32 capture for the bounding box of non-transparent pixels
  (background is injected transparent, so alpha = widget extent). Crop to that rect + Stretch=Fill
  maps the widget flush under the selection box: full-canvas widgets fall through to the full frame
  (identical to the confirmed-good image); floating widgets get tight-fitted with no dead space.
- Removes QueryContentBoundsAsync + the JS script entirely — the DOM-union attempt (a5b9952) broke
  rendering by measuring an unsettled layout on NavigationCompleted; alpha measurement is immune.
- 3 new unit tests: tight box, no-alpha no-crop, full-alpha no-crop. 7/7 pass, 0 warnings.

HANDOFF.md

@@ -6,34 +6,36 @@
 working). **NOT pushed — user rule: never push without explicit instruction (2026-08-24).** No feature
 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
+## The web-source rendering work (2026-08-28) — CURRENT MODEL: canvas-size viewport + ALPHA-BBOX crop
 
-**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.
+**History of failures (see MyMistakes.md for the full log):** box-size rendering clipped full-bleed
+widgets to their upper-left chunk (fixed — page renders at the master canvas 1920×1080, stable, the
+one render the creator confirmed solid). Then four crop attempts failed in different ways: (1) the
+`JSON.stringify` return double-encoded under `ExecuteScriptAsync`, silently disabling the crop;
+(2) `scrollWidth/scrollHeight` at canvas size returns the whole viewport for full-canvas wrapper
+bodies → no trim (ghost boundary); (3) union-of-DOM-elements measured an unsettled layout on
+NavigationCompleted and BROKE the image (creator's "don't break the rendering" boundary crossed).
+(4) Reverting to f39ebd9's scroll-crop was another dead end (#2 again).
 
-**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.**
+**The CURRENT MODEL (this commit):**
+- `Services/WebView2Manager.cs` — page renders at 1920×1080 (stable, never tracked); `overflow:hidden`
+  + transparent-bg injection. `FindContentBounds` scans the Bgra32 capture per frame and crops to the
+  bounding box of NON-TRANSPARENT pixels — the widget's true rendered extent, measured from the frame
+  itself. No DOM query, no layout timing: it can never truncate visible content (worst case = full
+  frame = the confirmed-good image). `Stretch="Fill"` maps the cropped frame flush under the selection
+  box → the box hugs the widget on all four sides. `QueryContentBoundsAsync` + the JS script are
+  DELETED.
+- `MainWindow.xaml` — web `<Image>` Stretch set to Fill via the IsWebSource DataTrigger (unchanged).
+- Invariant: **render at canvas size; crop to the alpha extent; Fill to the box.**
+- Unit tests: 3 new for `FindContentBounds` (tight box, no-alpha no-crop, full-alpha no-crop).
 
 **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"`
 restores click-anywhere-on-bar. All three confirmed fixed by the creator.
 
-Build: 0 warnings; tests 4/4. Rejected for the record: box-sized viewport (upper-left truncation),
-fixed 1920×1080 + `Uniform` (letterbox dead space), CSS margin/100% reset (enlarged gap),
-viewport==box (reflow/truncation/scrollbars).
+Build: 0 warnings; tests 7/7 (WebView2Manager). Rejected for the record: box-sized viewport (upper-left
+truncation), fixed 1920×1080 + `Uniform` (letterbox dead space), CSS margin/100% reset (enlarged gap),
+viewport==box (reflow/truncation/scrollbars), DOM-union crop (broke the image).
 
 ## Key facts / landmines
 
@@ -50,7 +52,8 @@ viewport==box (reflow/truncation/scrollbars).
 - No `CODING.md`; conventions live in `AGENTS.md` + `ai.md`.
 
 ## Next steps
-1. AWAIT creator's verdict on viewport==box (geometry-agnostic) rendering — visual check with a
-   widget URL at several box sizes; the selection frame must snap flush with no padding.
+1. AWAIT creator's verdict on the ALPHA-BBOX crop — reload a widget URL at several box sizes; the
+   selection frame must snap flush to the widget with no dead space, image must stay identical for
+   full-bleed widgets. If an animated widget causes visible jitter, add crop hysteresis as a follow-up.
 2. Follow-up only if the creator asks: graceful handling of navigation errors / invalid URIs (TASK 17 ×9).
 3. Continue queue: TASK 18 local recording, TASK 21 media source, TASK 22 audio sync offset.