Setting the width of a Google Docs Add-on sidebar doesn't work

Question

I'm creating a custom sidebar Google Docs add-on, but can't set its width. I have not published the addon as a web app.

Here's the code:

function onOpen(e) {
  DocumentApp.getUi()
      .createAddonMenu()
      .addItem('Show sidebar', 'showSidebar')
      .addToUi();
}

function showSidebar() {
  var html = HtmlService.createHtmlOutput('<b>Hello!</b>')
      .setSandboxMode(HtmlService.SandboxMode.IFRAME)
      .setTitle('How wide is this?')
      .setWidth(600);  // or 200; neither works - width is always 300px
  DocumentApp.getUi().showSidebar(html);
}

And here's what I get: always a 300-px wide sidebar:

Here's the test document

Answer 1

As mentioned in this page, sidebars shown by scripts are 300 pixels wide.