scheib/background.js

## background.js
chrome.app.runtime.onLaunched.addListener(function() {
  chrome.app.window.create('window.html');
});

## manifest.json
{
  "name": "onload focus test",
  "description": "Test that when a window is created it can have focus.",
  "version": "1",
  "manifest_version": 2,
  "app": {
    "background": {
      "scripts": ["background.js"]
    }
  },
  "permissions": [ ]
}

## window.html
<html>
<body>
  <p>When this app is opened the text area below should be focused and able to
  receive text. Note that this works when loading this page window.html in
  a browser.</p>
  <textarea>Initial text.</textarea>
<script src="window.js"></script>
</body>
</html>

## window.js
window.onload = function () {
  document.querySelector("textarea").value = "Type here.";
  document.querySelector("textarea").focus();
}
	chrome.app.runtime.onLaunched.addListener(function() {
	chrome.app.window.create('window.html');
	});
	{
	"name": "onload focus test",
	"description": "Test that when a window is created it can have focus.",
	"version": "1",
	"manifest_version": 2,
	"app": {
	"background": {
	"scripts": ["background.js"]
	}
	},
	"permissions": [ ]
	}
	<html>
	<body>
	<p>When this app is opened the text area below should be focused and able to
	receive text. Note that this works when loading this page window.html in
	a browser.</p>
	<textarea>Initial text.</textarea>
	<script src="window.js"></script>
	</body>
	</html>
	window.onload = function () {
	document.querySelector("textarea").value = "Type here.";
	document.querySelector("textarea").focus();
	}