title: PostMessage not received when application is hidden from taskbar
body: <p>I use PostMessage to send message to my application if another instance is trying to open:
  (<code>CUSTOMTEXT</code> replaced with my appname)</p>
  
  <pre><code>NativeMethods.PostMessage((IntPtr)NativeMethods.HWND_BROADCAST, NativeMethods.WM_CUSTOMTEXT_SHOWME, IntPtr.Zero, IntPtr.Zero);
  </code></pre>
  
  <p>And in WndProc I receive the message:</p>
  
  <pre><code>protected override void WndProc(ref Message m)
  {
      if (m.Msg == NativeMethods.WM_CUSTOMTEXT_SHOWME)
      {
          MessageBox.Show("Message received");
      }
  
      base.WndProc(ref m);
  }
  </code></pre>
  
  <p>And NativeMethods class:</p>
  
  <pre><code>class NativeMethods
  {
      public const int HWND_BROADCAST = 0xffff;
      public static readonly int WM_CUSTOMTEXT_SHOWME = RegisterWindowMessage("WM_CUSTOMTEXT_SHOWME");
  
      [DllImport("user32")]
      public static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam);
  
      [DllImport("user32")]
      public static extern int RegisterWindowMessage(string message);
  }
  </code></pre>
  
  <p>Everything works fine, but when I hide my application from taskbar (<code>this.ShowInTaskbar = false;</code>) my application stops receiving that message. </p>
  
  <p>Why? Is there any workaround for this?</p>