Doesn't appear "Not Responding" message on the form Title bar.

Hello,

In .net framework 3.5 and down:

I’ trying to imitate the  not responding UI thread. 

In the test project, I’m running simple window form and the process is stuck (sleep for 15000 for example) when I’m clicking on UI - "Not Responding" text is appear on the title bar.

In 3.5 and up the message doesn’t appear, do you know something about this strange phenomenon?