@borvik/dialog-polyfill

This is a fork of https://github.com/GoogleChrome/dialog-polyfill. Because of the polyfill nature, I'm fairly sure (but not certain) the feature I wanted would not be added (or merged back - and rightly so) as it would go beyond the polyfill it was designed to be.

Essentially I wanted to transform an ordinary <div> into a dialog so I could avoid the <dialog> stacking context and have a element with a z-index greater than the dialog show on top of it (think custom select/context menu or other dropdown).

dialog-polyfill.js is a polyfill for <dialog> and <form method="dialog">. Check out some demos!

<dialog> is an element for a popup box in a web page, including a modal option which will make the rest of the page inert during use. This could be useful to block a user's interaction until they give you a response, or to confirm an action. See the HTML spec.

Usage

Installation

You may optionally install via NPM -

$ npm install dialog-polyfill

There are several ways that to include the dialog polyfill:

include dist/dialog-polyfill.js script directly in your HTML, which exposes a global dialogPolyfill function.
import (es modules)
require (commonjs/node)

// direct import (script module, deno)
import dialogPolyfill from './node_modules/dialog-polyfill/dist/dialog-polyfill.esm.js';

// *OR*

// modern es modules with rollup/webpack bundlers, and node via esm module
import dialogPolyfill from 'dialog-polyfill'

// *OR*

// traditional commonjs/node and browserify bundler
const dialogPolyfill = require('dialog-polyfill')

Supports

This polyfill works on modern versions of all major browsers. It also supports IE9 and above. It can work when used inside Shadow DOM, but it's not recommended.

Steps

Include the CSS in the <head> of your document, and the JS anywhere before referencing dialogPolyfill.
Create your dialog elements within the document. See limitations for more details.
Register the elements using dialogPolyfill.registerDialog(), passing it one node at a time. This polyfill won't replace native support.
Use your <dialog> elements!

Script Global Example

<head>
  <link rel="stylesheet" type="text/css" href="dist/dialog-polyfill.css" />
</head>
<body>
  <dialog>
    I'm a dialog!
    <form method="dialog">
      <input type="submit" value="Close" />
    </form>
  </dialog>
  <script src="dist/dialog-polyfill.js"></script>
  <script>
    var dialog = document.querySelector('dialog');
    dialogPolyfill.registerDialog(dialog);
    // Now dialog always acts like a native <dialog>.
    dialog.showModal();
  </script>
</body>

::backdrop

In native <dialog>, the backdrop is a pseudo-element. When using the polyfill, the backdrop will be an adjacent element:

dialog::backdrop { /* native */
  background-color: green;
}
dialog + .backdrop { /* polyfill */
  background-color: green;
}

Limitations

In the polyfill, modal dialogs have limitations-

They should not be contained by parents that create a stacking context, see below
The browser's chrome may not always be accessible via the tab key
Changes to the CSS top/bottom values while open aren't retained

Stacking Context

The major limitation of the polyfill is that dialogs should not have parents that create a stacking context. The easiest way to solve this is to move your <dialog> element to be a child of <body>.

If this isn't possible you may still be able to use the dialog. However, you may want to resolve it for two major reasons-

The polyfill can't guarantee that the dialog will be the top-most element of your page
The dialog may be positioned incorrectly as they are positioned as part of the page layout where they are opened (defined by spec), and not at a fixed position in the user's browser.

To position a dialog in the center (regardless of user scroll position or stacking context), you can specify the following CSS-

dialog {
  position: fixed;
  top: 50%;
  transform: translate(0, -50%);
}

This is also provided as a helper CSS class in the polyfill CSS, .fixed. You can apply by using HTML like <dialog class="fixed">.

Extensions

Focus

The WAI-ARIA doc suggests returning focus to the previously focused element after a modal dialog is closed. However, this is not part of the dialog spec itself. See this snippet to add this, even to the native dialog.