gecko-dev/toolkit/content/tests/chrome/file_edit_contextmenu.xhtml

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="chrome://global/skin/global.css"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
       xmlns:html="http://www.w3.org/1999/xhtml">
<script>
customElements.define("shadow-input", class extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: "open" });
    this.shadowRoot.appendChild(document.createElement("input"));
  }
});
</script>
<script type="application/javascript" src="chrome://global/content/globalOverlay.js"/>
<!-- Copied from toolkit/content/editMenuCommands.inc.xul -->
<script type="application/javascript" src="chrome://global/content/editMenuOverlay.js"/>
<commandset id="editMenuCommands">
  <commandset id="editMenuCommandSetAll" commandupdater="true" events="focus,select"
              oncommandupdate="goUpdateGlobalEditMenuItems()"/>
  <commandset id="editMenuCommandSetUndo" commandupdater="true" events="undo"
              oncommandupdate="goUpdateUndoEditMenuItems()"/>
  <commandset id="editMenuCommandSetPaste" commandupdater="true" events="clipboard"
              oncommandupdate="goUpdatePasteMenuItems()"/>
  <command id="cmd_undo" oncommand="goDoCommand('cmd_undo')"/>
  <command id="cmd_redo" oncommand="goDoCommand('cmd_redo')"/>
  <command id="cmd_cut" oncommand="goDoCommand('cmd_cut')"/>
  <command id="cmd_copy" oncommand="goDoCommand('cmd_copy')"/>
  <command id="cmd_paste" oncommand="goDoCommand('cmd_paste')"/>
  <command id="cmd_delete" oncommand="goDoCommand('cmd_delete')"/>
  <command id="cmd_selectAll" oncommand="goDoCommand('cmd_selectAll')"/>
  <command id="cmd_switchTextDirection" oncommand="goDoCommand('cmd_switchTextDirection');"/>
</commandset>

<menupopup id="outer-context-menu">
  <menuseparator id="customizeMailToolbarMenuSeparator"/>
  <menuitem id="hello" label="Hello" accesskey="H"/>
</menupopup>

<hbox context="outer-context-menu">
<html:textarea />
<html:input />
<search-textbox />
<html:shadow-input />
</hbox>

</window>