Under Settings for a Screen, you'll see a new Fit width option. The width of the screen will automatically adjust (from the width set in Editor) to width of the browser in Player. The height of screen will remain fixed to the height set in Editor. Use this setting to prototype fluid web pages. Note that with Fit to window selected, both the width and height of the screen will automatically adjust (from the size set in Editor) to the size of the browser in Player. Use this setting to prototype applications with independent scrolling panels. 

You can now size and position elements by percentage or pixel. Elements can also be positioned relative to any edge or corner, not just positioned from the upper left corner. New layout options control how the size and position of an individual widget will adjust as it's parent container resizes. For example, you can set an element to appear 10px from the bottom right corner of a section/screen, even as the section/screen is resized.