C# Winforms main form sent to back

Question

My Winforms app has a MainForm and a MainWaitForm. For some reason I don't understand, my MainForm keeps being sent behind other opened windows, mainly when users click on anything when the MainWaitForm is showing... For example, when running the app in VisualStudio debugger mode, the wait dialog shows but VisualStudio comes right behind, instead of leaving the MainForm right on second level.

Here is the code of the wait form overlay :

static Control _parent;
public static void ShowWaitForm(Control parent) {
    _parent = parent;
    // some attempt to bring the active view to front first...
    if (_parent != null && _parent.Visible) {
      _parent.BringToFront();
    }
    else {
      MyMainForm.BringToFront();
    }
    // Make sure it is only launched once.
    if (_instance != null) {
      return;
    }
    _thread = new Thread(new ParameterizedThreadStart(MainWaitForm.ShowForm));
    _thread.IsBackground = true;
    _thread.SetApartmentState(ApartmentState.STA);
    _thread.Start(parent);
    while (_instance == null || _instance.IsHandleCreated == false) {
    System.Threading.Thread.Sleep(100);
    }
}

// the hide method
 public static void HideWaitForm() {
  // again, attempt to bring form to front...
  if (_parent != null && _parent.Visible) {
      _parent.BringToFront();
  }
  else {
      MyMainForm.BringToFront();
  }
  if (_instance != null) {
    if (_instance.InvokeRequired) {
      _instance.Invoke(new System.Windows.Forms.MethodInvoker(delegate {
        CloseForm();
      }));
    }
    else {
      CloseForm();
    }
  }
}

static void CloseForm() {
  _instance.Close();
  _thread = null;
  _instance = null;
}