Hi Dave,

I ran your sample and I get the same results.

If I run the same sample in VS2010 or run it in VS2012 without the Debugger, it works fine. So this appears to be a bug in VS2012 with the Debugger.

I checked it out and it looks like this is a known bug and has been reported to Microsoft here:

WinForm dialogs displayed differently by *.vshost.exe and *.exe using VS 2012 when FormBorderStyle set in form codebehind | Microsoft Connect

There's nothing we can do about it, but the good news is that it only happens when the Debugger is attached, so it will not affect your clients using your app.

Microsoft posted a handful of workarounds within the link provided on the work item; one of which is specific to projects that target the .NET 4.5 Framework.

“If your are building a 4.5 application you would need to request the user to disable themes on the target machine or apply workaround #1.”

Workaround #1:

1. In order to achieve runtime parity between NETFX 4.0 and 4.5, when building a manually change SubSystemVersion compiler option in the project file targeting NETFX 4.5, to a value lower that 6.00, see this article for details:
   http://msdn.microsoft.com/en-us/library/hh965708.aspx
   For example: add the following property to your project file:
   "<subsystemversion>5.01</subsystemversion> "
   Note that 64bits does not support all subsystem versions numbers.