The UI in SharePoint Designer 2010 works differently from most development apps when dealing with code items like Site Assets and Master Pages.
In SPD, clicking on an item name (say, Home.aspx under 'Site Pages') will open a detail view (showing File Information, Customization, Permissions, Version History).
In Visual Studio, on the other hand, clicking an item in the Solution Explorer will simply select the item, and it requires a double-click to open the file.
Is there any way to make SPD work like Visual Studio, so that clicking an item selects it? Any development project seems to require constant flicking between VS, SPD, and the browser, and the different UI idioms drive me slightly nuts.