The Tor Browser / file revision

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         class="reftest-wait">

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       const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";

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       var vbox = document.getElementsByTagName("vbox")[0];

       var span = document.createElementNS(HTML_NS, "html:span");

       vbox.appendChild(span); // causes block wrapping

       setTimeout(finish, 0);

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     function finish() {

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     }

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   <vbox flex="1">

     <box flex="1" />

     <label value="hello world" />

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