Hi Jonatan,
As for the UserControl, do you mean a custom WinJS control built by yourself like the following article mentioned?
#Building a custom control using the Windows Library for JavaScript (WinJS)
http://blogs.msdn.com/b/windowsappdev/archive/2012/10/11/building-a-custom-control-using-the-windows-library-for-javascript-winjs.aspx
In Windows Store javascript application, it is recommended that we use CSS to control the layout of page/controls under different view state mode. Or if you want to handle layout change in code, you can add event listener to the "resize" event in the initialize
funciton of your WinJS control. e.g.
window.addEventListener("resize", onViewStateChanged);
here is a good article discussing on handling UI view state change in Windows Store javascript app:
#Handling Fullscreen, Snapped and Filled states in Windows 8 Metro Style apps using CSS3 and JavaScript
http://blogs.msdn.com/b/davedev/archive/2012/05/22/handling-fullscreen-snapped-and-filled-states-in-windows-8-metro-style-apps-using-css3-and-javascript.aspx
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