Web source: container == content — element grid + selection box sized to the cropped frame's canvas extent

- FindContentBounds already trimmed the bitmap; the grid/box still rendered at the uncropped user
  size, so Fill stretched the tight widget into a bigger frame = dead-between-content-and-border.
- ContentCanvasSize maps the crop rect to canvas units (1920/pixW per axis, DPI-correct) and the
  preview event now carries (bitmap, canvasW, canvasH).
- OnWebView2PreviewBitmapChanged sets the element Width/Height to that size; the grid and the
  SelectionOverlay (both bound to the element's Width/Height) now equal the widget exactly.
- IsPreviewDragging (set in MainWindow drag/resize) suppresses auto-fit mid-gesture.
- Tests 10/10 (alpha bounds, canvas-size mapping incl. DPI, control size); 0 warnings.

HANDOFF.md

@@ -6,36 +6,34 @@
 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 + ALPHA-BBOX crop
+## The web-source rendering work (2026-08-28) — CURRENT MODEL: canvas-size viewport + alpha crop + container==content
 
-**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).
+**History of failures (see MyMistakes.md for the full log):**
+1. box-sized viewport clipped full-bleed widgets to their upper-left chunk → render at the master
+   canvas 1920×1080 (stable), the only render the creator confirmed solid.
+2. `JSON.stringify` return double-encoded under `ExecuteScriptAsync` → crop silently disabled.
+3. `scrollWidth/scrollHeight` at canvas size = whole viewport for full-canvas wrapper bodies → no trim.
+4. DOM-element-union crop measured an unsettled layout on NavigationCompleted → BROKE the image.
+5. Alpha-bbox crop (from the frame itself) fixed the BITMAP but the grid/box stayed at the uncropped
+   user size → Fill stretched the tight widget into a big frame → dead space between content and border.
 
-**The CURRENT MODEL (this commit):**
+**The CURRENT MODEL (this change):**
 - `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).
+  + transparent-bg injection. `FindContentBounds` crops the Bgra32 capture to the bounding box of
+  non-transparent pixels each frame (immune to timing, can never truncate content). `ContentCanvasSize`
+  maps the crop rect to CANVAS UNITS (DPI-correct). Preview event now carries (bitmap, canvasW, canvasH).
+- `ViewModels/MainViewModel.cs` — `OnWebView2PreviewBitmapChanged` sets the element's Width/Height to
+  the cropped canvas size → the element grid AND the SelectionOverlay (both bound to element
+  Width/Height) EQUAL the widget. `IsPreviewDragging` suppresses the auto-fit during a drag gesture.
+- `MainWindow.xaml.cs` — sets/clears `IsPreviewDragging` at drag/resize start and end.
+- Invariant: **render at canvas size; crop to the alpha extent; SIZE THE BOX to the crop.**
+- Unit tests: 10/10 (alpha bounds ×3, canvas-size mapping ×3, control size ×4).
 
 **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.
+`IsTabStop="False"`; Slider style `IsMoveToPointEnabled="True"`. Confirmed fixed by the creator.
 
-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).
+Build: 0 warnings; tests 10/10. Rejected for the record: box-sized viewport, `Uniform`, CSS reset,
+viewport==box, DOM-union crop (broke image), crop-without-container-size (left the gap).
 
 ## Key facts / landmines
 
@@ -52,8 +50,8 @@ viewport==box (reflow/truncation/scrollbars), DOM-union crop (broke the image).
 - No `CODING.md`; conventions live in `AGENTS.md` + `ai.md`.
 
 ## Next steps
-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).
+1. AWAIT creator's verdict on the container==content auto-fit — reload a widget; the selection frame
+   must equal the widget exactly (no dead space, including during resize). Follow-ups only if asked:
+   crop hysteresis for animated widgets, ticker/percent-display widgets.
+2. Graceful handling of navigation errors / invalid URIs (TASK 17 ×9) — only if the creator asks.
 3. Continue queue: TASK 18 local recording, TASK 21 media source, TASK 22 audio sync offset.