Fix fresh browser screenshot prep race

docs/browser-capture-harness.md

@@ -8,6 +8,8 @@ It validates the compositor behavior that unit tests cannot see:
 - 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;
+- a newly attached resident webview can be captured immediately after load, without
+  an extra settle delay;
 - 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.
 
@@ -25,9 +27,10 @@ packages/desktop/capture-harness/out/
 
 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.
+prep captures and five full-page prep captures for each of the first two webviews, a
+fresh-immediate viewport and full-page capture for a newly attached webview, 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