test(browser): draw the page before holding a dialog in the pane test

docs/ceilings.md

@@ -1002,6 +1002,12 @@ work when nobody knows it and that the call site never shows. The mechanism and
   sibling covers keeps its pin. Notes not sent yet
   live in the window: closing it, or the browser, loses them.
 
+- **On macOS, a JavaScript dialog a page opens before its first paint leaves the pane black behind the dialog's
+  card** (`internal/browser/chrome_view.go`): headless Chrome there paints a page some time after its load, and
+  a page held by a dialog cannot paint, so a screencast started then has no frame to send until the dialog is
+  answered (Chrome 155 on the macos-latest runner, measured). A dialog opened once the page has been drawn, as
+  one an agent's click opens, shows the page behind it. Linux and Windows paint before the load returns.
+
 - **A session browser's downloads go straight to the user's Downloads folder** (`internal/browser/downloads.go`,
   `session_downloads.go`, `docs/browser-tab.md`): `XDG_DOWNLOAD_DIR` on Linux, `~/Downloads` on macOS and Windows,
   even for a Windows user who moved that known folder. Chrome writes each one under its download id into a staging

internal/browser/dialogs_live_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/chromedp/cdproto/page"
 	"github.com/chromedp/chromedp"
 	"github.com/lichdotdev/lich/api"
 	"github.com/lichdotdev/lich/internal/chromium"
@@ -105,6 +106,16 @@ func TestChromeAPaneOpenedOnADialogShowsThePage(t *testing.T) {
 	if err := p.Navigate(ctx, srv.URL); err != nil {
 		t.Fatal(err)
 	}
+	// The page is drawn once first, as a page the agent clicked on was:
+	// headless Chrome on macOS has not painted it when Navigate returns, and a
+	// dialog held open before its first paint leaves no frame to cast
+	// (Chrome 155, macos-latest runner, measured 2026-10-28).
+	if err := p.runFree(ctx, chromedp.ActionFunc(func(ctx context.Context) error {
+		_, err := page.CaptureScreenshot().Do(ctx)
+		return err
+	})); err != nil {
+		t.Fatal(err)
+	}
 	if err := p.runFree(ctx, chromedp.Evaluate(`setTimeout(() => confirm('Delete?'), 0)`, nil)); err != nil {
 		t.Fatal(err)
 	}