Fix multi-webview browser capture prep

docs/browser-capture-harness.md

@@ -6,6 +6,8 @@ It validates the compositor behavior that unit tests cannot see:
 - the resident automation `<webview>` starts in the production parking state;
 - the parked guest has no copyable viewport frame;
 - the resident webview guest is sized to 1280x800 logical pixels;
+- multiple resident webviews are parked as an overlapping stack, and the capture
+  target is raised above its sibling webviews before capture;
 - the app-style prep sequence moves the host to a paintable 1x1 clipped state, waits two animation frames and a layout read, then both viewport `capturePage` and full-page CDP screenshots return real pixels;
 - restore returns the host to offscreen parking after every capture.
 
@@ -21,10 +23,11 @@ The harness writes PNG evidence and `results.json` to:
 packages/desktop/capture-harness/out/
 ```
 
-A passing run prints `PASS` lines for guest sizing, the expected parked-capture failure,
-five viewport prep captures, five full-page prep captures, and final completion. The PNG
-sizes may be device-pixel scaled; on a Retina display the 1280x800 logical viewport is
-usually saved as 2560x1600.
+A passing run prints `PASS` lines for both guest sizes, the expected parked-capture
+failure, the legacy stacked-below-the-clip failure for the second webview, five viewport
+prep captures and five full-page prep captures for each of the first two webviews, and
+final completion. The PNG sizes may be device-pixel scaled; on a Retina display the
+1280x800 logical viewport is usually saved as 2560x1600.
 
 ## Mechanism
 
@@ -34,6 +37,10 @@ webview parked at `left:-20000px` and `opacity:0` does not have a copyable surfa
 
 Before pixel capture, the app renderer temporarily makes the resident host paintable:
 `left:0`, `top:0`, `opacity:1`, `pointer-events:none`, host size `1x1`, and
-`overflow:hidden`, with the full-size 1280x800 webview inside. Main captures only after
-the renderer acknowledges two animation frames plus a `getBoundingClientRect()` read, and
-the renderer restores parking in a `finally`.
+`overflow:hidden`, with the full-size 1280x800 webview inside. Resident webviews are
+parked absolutely at `0,0` inside that host because a second webview stacked below the
+1px clip still has no copyable surface. During capture, the renderer raises the target
+webview above the other resident webviews; an overlay or sibling above the target can make
+full-page CDP capture fail. Main captures only after the renderer acknowledges two
+animation frames plus a `getBoundingClientRect()` read, and the renderer restores parking
+in a `finally`.