Extendscript: Gets currently open comp from timeline, as opposed to selected comp in project panel

getFocusedComp.jsx

(function () {
  /**
   * Gets currently open comp from timeline, as opposed to
   * selected comp in project panel
   *
   * @returns {CompItem | null} Active comp, or null if none
   */
  function getFocusedComp() {
    var activeItem = app.project.activeItem;

    // If there is no active item, or it's not a comp, return null.
    if (!(activeItem && activeItem instanceof CompItem)) {
      return null;
    }

    // If there are multiple items selected in the project panel,
    // activeItem will always be the active comp -- so we found it.
    if (app.project.selection.length !== 1) {
      return activeItem;
    }

    // If there's only one, let's add a null to the layer and see
    // whether the null was added to the same comp as activeItem
    // If so, then we know that the focused comp is the activeItem
    var numLayers = activeItem.numLayers;

    app.executeCommand(2767); // 'add null object'

    if (numLayers !== activeItem.numLayers) {
      app.executeCommand(16); // undo (removes 'null object')
      return activeItem;
    }
  }

  var focusedComp = getFocusedComp();

  if (focusedComp === null) {
    $.writeln("No comp in focus! Assume focus is project panel.");
  } else {
    $.writeln("Focused comp: " + focusedComp.name);
  }
})();