fune/browser/components/preferences/in-content/tests/browser_containers_name_input.js

const CONTAINERS_URL = "chrome://browser/content/preferences/containers.xul";

add_task(async function setup() {
  await openPreferencesViaOpenPreferencesAPI("containers", { leaveOpen: true });
  registerCleanupFunction(async function() {
    await BrowserTestUtils.removeTab(gBrowser.selectedTab);
  });
});

add_task(async function() {
  async function openDialog() {
    let doc = gBrowser.selectedBrowser.contentDocument;

    let dialogPromise = promiseLoadSubDialog(CONTAINERS_URL);

    let addButton = doc.getElementById("containersAdd");
    addButton.doCommand();

    let dialog = await dialogPromise;

    return dialog.document;
  }

  let doc = await openDialog();

  let name = doc.getElementById("name");
  let btnApplyChanges = doc.getElementById("btnApplyChanges");

  Assert.equal(name.value, "",
               "The name textbox should initlally be empty");
  Assert.ok(btnApplyChanges.disabled,
            "The done button should initially be disabled");

  function setName(value) {
    name.value = value;

    let event = new doc.defaultView.InputEvent("input", { data: value });
    SpecialPowers.dispatchEvent(doc.defaultView, name, event);
  }

  setName("test");

  Assert.ok(!btnApplyChanges.disabled,
            "The done button should be enabled when the value is not empty");

  setName("");

  Assert.ok(btnApplyChanges.disabled,
            "The done button should be disabled when the value is empty");
});