How to make WPF pop-up windows not be hidden behind main application?

Answer 1

To arrange windows in a visual hierarchy you have to set the Owner property of the child window to the parent window.

You can read more about the Owner property on MSDN.

You should change your code into something similar to this:

Window parentWindow;

private void Button_OpenFirst(object sender, RoutedEventArgs e)
{
  this.parentWindow = new Window();
  this.parentWindow.Owner = this;
  this.parentWindow.Show();
}

private void Button_OpenSecond(object sender, RoutedEventArgs e)
{
  Window childWindow = new Window();
  childWindow.Owner = this.parentWindow;
  childWindow.Show();
}