Why does closing the last child window minimize its parent window?

Question

I have the following simple wpf application:

App.xaml:

<Application x:Class="TestWpf2.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
</Application>

App.xaml.cs:

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        var parentWindow = new Window();
        parentWindow.Show();

        var childWindow1 = new Window { Owner = parentWindow };
        childWindow1.Show();

        var childWindow2 = new Window { Owner = parentWindow };
        childWindow2.Show();
    }
}

The application causes 3 windows to appear on screen. If you run the application and close the two child windows, the parent window is minimized to the task bar. If you comment out childWindow2.show(), run the application, and close the single child window, the parent window is not minimized to the taskbar.

I can add the following code to work around this problem:

childWindow1.Closing += delegate(object sender, CancelEventArgs ex)
{
    (sender as Window).Owner = null;
};

but I don't want to use a hack like this, and I want to understand why this problem occurs.

Why does this happen?

It is not minimized, it just loses the focus and hides behind another window. Like Visual Studio's main window. Adding ShowActivated = false to the child window initializer is one way to fix it. — Hans Passant
@HansPassant ShowActivated only stops the child windows from being activated when they are created. If they user clicks on them to activate them, then closes them, the main window still loses focus. — Oliver
connect.microsoft.com/VisualStudio/feedback/details/523197/… — Hans Passant
If you add a parentWindow.Activate() after having created child windows, it should fix the problem. Child windows steal activation (input) and when no window is activated, another window gets it. — Simon Mourier

CharithJ CharithJ · Accepted Answer · 2017-06-13T13:17:17

This is a WPF undocumented feature(Bug)

This bug has been reported to Microsoft more than 7 years ago.

Modal dialog on top of non-modal window sends main window to back.

The WPF team has recently reviewed this issue and will not be addressing this issue as at this time the team is focusing on the bugs impacting the highest number of WPF developers.

Let's look at the behavior of this undocumented feature a bit.

It doesn't Minimize, just go one window behind (in debug mode, behind the visual studio window)

Steps to prove:

Run this application.
Minimize all other windows (eg: visual studio and all other windows). Keep only these three windows on the screen.
Close both children, parent is still in Normal State

Test this code for further proof: StateChange wouldn't fire.

        protected override void OnStartup(StartupEventArgs e)
        {
            var parentWindow = new Window() { Title = "Parent" };
            parentWindow.StateChanged += (sender, ep)=>
            {
                var state = ((Window)sender).WindowState;
            };
            parentWindow.Show();

            var childWindow1 = new Window { Owner = parentWindow, Title = "Child1" };
            childWindow1.Show();

            var childWindow2 = new Window { Owner = parentWindow, Title = "Child2" };
            childWindow2.Show();
        }

Being TopMost prevent this happening

If parent window is TopMost, this wouldn't happen. But this may not be an option in many cases.

parentWindow.Topmost = true;

Workaround

Activate the parent on child2 close.

childWindow2.Closed += (a, b) => { parentWindow.Activate(); };

Why does closing the last child window minimize its parent window?

3 Answers