The terrible story about .NET Invoke

1.  There is something interesting about PostMessage

There is a limit of 10,000 posted messages per message queue. This limit should be sufficiently large. If your application exceeds the limit, it should be redesigned to avoid consuming so many system resources. To adjust this limit, modify the following registry key.

HKEY_LOCAL_MACHINE
   SOFTWARE
      Microsoft
         Windows NT
            CurrentVersion
               Windows
                  USERPostMessageLimit

The minimum acceptable value is 4000. 

https://msdn.microsoft.com/ru-ru/library/windows/desktop/ms644944%28v=vs.85%29.aspx

2.  And this explains why Invoke hangs when reaches the limit:

private object MarshaledInvoke(Control caller, 
                               Delegate method, 
                               object[] args, 
                               bool synchronous)
{
    int lpdwProcessId;
/// ....
/// skipped
/// ....
    if (flag)
        this.InvokeMarshaledCallbacks();
    else
    {
        UnsafeNativeMethods.PostMessage(new HandleRef(this, this.Handle), 
                                        threadCallbackMessage, 
                                        IntPtr.Zero, 
                                        IntPtr.Zero);
    }
    if (!synchronous)
        return entry;
    if (!entry.IsCompleted)
        this.WaitForWaitHandle(entry.AsyncWaitHandle); // <<<< OOPS
    if (entry.exception != null)
        throw entry.exception;
    return entry.retVal;
}

http://workblog.pilin.name/2007/04/control.html 

It's a scary, scary world