In https://chromium-review.googlesource.com/c/chromium/src/+/1188643, timothygu@ and alexmos@ spotted a window.open case where GuestViews seem to depend on the old OpenURL path in the renderer rather than the BeginNavigation path.

More specifically, in the WebViewNewWindowInteractiveTest.NewWindow_Redirect test in interactive_ui_tests, a <webview> tries to open a new window using this approach:

        var w = window.open('', frameName);
        w.opener = null;
        w.document.write(
            '<META HTTP-EQUIV="refresh" content="0; url=' + url + '">');
        w.document.close();

That models what Gmail was doing in issue 233475.  The "w=window.open();w.opener=null;navigate" part triggers the "Gmail fork heuristic" where OpenURL is called, and the document.write + meta-refresh part is what Gmail was using to navigate.

Fady fixed this case for <webview>, but Alex found that it seems to depend on the OpenURL path.  We're moving away from that path in general now that PlzNavigate has shipped, and timothygu@'s https://chromium-review.googlesource.com/c/chromium/src/+/1188643 happened to also change the document.write behavior so that OpenURL also doesn't kick in (since the old_url in decidePolicyForNavigation no longer looks like about:blank after document.write, basically undoing the fix for  issue 93517 ).

We should update GuestView to support opening windows in this way using BeginNavigation instead.  As Alex notes:

https://chromium-review.googlesource.com/c/chromium/src/+/1188643/8/chrome/test/data/extensions/platform_apps/web_view/newwindow/guest_opener.html#30
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In the case with BeginNavigation, the navigation gets canceled by a new about:blank that originates from here in WebViewGuest::ApplyAttributes: https://cs.chromium.org/chromium/src/extensions/browser/guest_view/web_view/web_view_guest.cc?l=1154&rcl=630ab6596e3620e984fe6a45e82a23b07600b5c4

Looks like that function doesn't read the correct URL from pending_new_windows_, and indeed, it depends on the OpenURL branch putting it there in the first place, via WebViewGuest::OpenURLFromTab -> WebViewGuest::CreateNewGuestWebViewWindow -> WebViewGuest::NewGuestWebViewCallback.  So it does appear that window.open from a <webview> is only supported via OpenURL.  I think it's probably possible to fix this to also work with the PlzNavigate path (BeginNavigation)
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We'd like to proceed with Timothy's CL, so hopefully we can fix this in parallel in M71 to avoid a possible regression.  James, can you help triage and find an owner?