Windows 8 / IE10: Browser window, opened from another browser window, calls alert() function and loses focus

IE browser window calls alert() and loses focus if it is opened from another window. (only in IE10 / Windows 8)

Here's the code to reproduce the problem.

Save this as 'parent.html.'

<input type="button" 
name="test" 
value="test" 
onclick="window.open('child.html','childwindow','width=800,height=400')" >

And save this as 'child.html'

<input type="text" name="test" value="here" onfocus="alert('test')">

Open parent.html with IE and click the button. Another window opens and click the input text.

Then the new window loses focus.

Is this bug in IE10 ? Any workarounds ?