The Tor Browser / file revision

 <?xml version="1.0"?>

 <!DOCTYPE window>

 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" title="RTL overflow test" width="640px" height="480px" style="direction: rtl" onload="load()" class="reftest-wait">

 <hbox style="max-width: 200px; border: 1px solid red;">

 <scrollbox id="sb" align="start" pack="start" orient="horizontal" flex="1">

   <button id="sb1" label="test1"/><button label="test2"/><button label="test3"/><button label="test4"/><button label="test5"/>

 </scrollbox>

 </hbox>

 <hbox style="max-width: 200px; border: 1px solid blue">

 <arrowscrollbox id="asb" align="start" pack="start" orient="horizontal" flex="1">

   <button id="asb1" label="test1"/><button label="test2"/><button label="test3"/><button label="test4"/><button label="test5"/>

 </arrowscrollbox>

 </hbox>

 <hbox style="max-width: 200px; border: 1px solid black">

 <hbox align="end" pack="end" flex="1">

   <button label="test1"/><button label="test2"/><button label="test3"/><button label="test4"/><button label="test5"/>

 </hbox>

 </hbox>

 <script type="text/javascript">

   <![CDATA[

     function load() {

       // arrowscrollbox provides a helper ensureElementIsVisible method

       document.getElementById("asb").ensureElementIsVisible(document.getElementById("asb1"), false);

       // scrollbox doesn't provide that, so we have to do the heavy lifting directly

       var sb = document.getElementById("sb");

       var sb1 = document.getElementById("sb1");

       var rect = sb.getBoundingClientRect();

       var containerStart = rect.left;

       var containerEnd = rect.right;

       rect = sb1.getBoundingClientRect();

       var elementStart = rect.left;

       var elementEnd = rect.right;

       var amountToScroll = 0;

       if (elementStart < containerStart) {

         amountToScroll = elementStart - containerStart;

       } else if (elementEnd > containerEnd) {

         amountToScroll = elementEnd - containerEnd;

       }

       sb.scrollLeft += amountToScroll;

       document.documentElement.removeAttribute("class");

     }

   ]]>

 </script>

 </window>