This is a pretty nasty bug that was introduced in .NET 4.5. The KB article is available here. The fix is right now only available as a hotfix, hopefully it will make it into a service update soon. I'll just copy/paste the description:
Assume that you have a .NET Framework 4.5-based Windows Form application. When you click a menu item to open a child window in the application, interactions with the menu and child window behave incorrectly.
For example, you may experience the following:
When you open a context menu in the child window, the main window takes the focus.
You cannot use mnemonics to access a menu item.This issue occurs because the IMessageFilter interface is unhooked too aggressively. Therefore, the .NET Framework 4.5 does not filter menu-related window messages.
Update: this issue was fixed in a .NET 4.5 update released on Jan 8th, 2013. The KB article is here.