gecko-dev/browser/base/content/test/general/browser_drag.js

function test() {
  waitForExplicitFinish();

  let EventUtils = {};
  Services.scriptloader.loadSubScript(
    "chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
    EventUtils
  );

  // ---- Test dragging the proxy icon ---
  var value = content.location.href;
  var urlString = value + "\n" + content.document.title;
  var htmlString = '<a href="' + value + '">' + value + "</a>";
  var expected = [
    [
      { type: "text/x-moz-url", data: urlString },
      { type: "text/uri-list", data: value },
      { type: "text/plain", data: value },
      { type: "text/html", data: htmlString },
    ],
  ];
  // set the valid attribute so dropping is allowed
  var oldstate = gURLBar.getAttribute("pageproxystate");
  gURLBar.setAttribute("pageproxystate", "valid");
  var dt = EventUtils.synthesizeDragStart(
    document.getElementById("identity-box"),
    expected
  );
  is(dt, null, "drag on proxy icon");
  gURLBar.setAttribute("pageproxystate", oldstate);
  // Now, the identity information panel is opened by the proxy icon click.
  // We need to close it for next tests.
  EventUtils.synthesizeKey("VK_ESCAPE", {}, window);

  // now test dragging onto a tab
  var tab = BrowserTestUtils.addTab(gBrowser, "about:blank", {
    skipAnimation: true,
  });
  var browser = gBrowser.getBrowserForTab(tab);

  browser.addEventListener(
    "load",
    function() {
      is(
        browser.contentWindow.location,
        "http://mochi.test:8888/",
        "drop on tab"
      );
      gBrowser.removeTab(tab);
      finish();
    },
    true
  );

  EventUtils.synthesizeDrop(
    tab,
    tab,
    [[{ type: "text/uri-list", data: "http://mochi.test:8888/" }]],
    "copy",
    window
  );
}