Wondering if anybody else has found some very odd behaviour with the XAML/WPF 4 editor in VS2010. This only occurs if the project is using .NET 4. Whenever I tried to open a XAML document for editing, the window would appear to open for a split second and then vanish, but VS2010 would still list the window as open. The fault was eventually traced to having the "Reuse current document window, if saved" option under "Documents" in the "Environment" options checked. Once this was unchecked XAML 4 files opened as expected. As I said, this only appears to occur on projects targeted at .NET Framework 4 - those targeted at 3.5 worked without a problem, and the "Reuse current document window, if saved" appears to work fine on other files.