form.ShowDialog(owner) with owner that is Topmost stays hidden behind the owner

[fivage]

• .NET Core Version: 5

• Have you experienced this same bug with .NET Framework?: No, works correctly in 4.7.2

Problem description:

form.ShowDialog(owner) with owner that is Topmost stays hidden behind the owner.

Expected behavior:

It should be put in front of the owner

[RussKie]

Please provide a repro that shows the issue. Also an animation of the issue would help.

[ghost]

This submission has been automatically marked as stale because it has been marked as requiring author feedback but has not had any activity for 14 days.

It will be closed if no further activity occurs within 7 days of this comment.

[fivage]

WinFormsApp2.zip

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            this.TopMost = true;
        }

        private void OpenPopupButton_Click(object sender, EventArgs e)
        {
            PopupForm popupForm = new();
            popupForm.ShowDialog(this);  // PopupForm should be in front of Form1, but is positioned behind Form1 
        }
    }

[hyperquantum]

A workaround seems to be to set TopMost = true in the child window if it detects that its parent has TopMost enabled.

[merriemcgaw]

@hyperquantum that looks like a reasonable workaround. @Olina-Zhang can you help us figure out where this one changed? Was it working in 3.1 the same as it did in 4.7.2/4.8?

[Olina-Zhang]

@merriemcgaw verified this issue in .Net Core 3.1, it works well, and it repro in .Net 5.0/6.0/7.0.

Here is a gif in .Net Core 3.1 behavior:

[elachlan]

Related #9351

[DinkoK]

Hello, We have encountered a similar behavior when using a derivative of MS CommonDialog. In this case, our Form has the TopMost property set to true. When CommonDialog is shown it will be placed under the Form. It can be reproduced using the following code:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.TopMost = true;
    }

    private void radButton1_Click_1(object sender, EventArgs e)
    {
        new MyCommonDialog().ShowDialog();
    }
}

public class MyCommonDialog : CommonDialog
{
    private Form myForm = new Form();

    public override void Reset()
    {
    }

    protected override bool RunDialog(IntPtr hwndOwner)
    {
        return ((Form)myForm).ShowDialog(NativeWindow.FromHandle(hwndOwner)) == DialogResult.OK;
    }
} 

This behavior is not observed in NetCore 3.1. It is reproducible in Net 5 and above.

I am also attaching my test project.

Show_TopMost.zip

[sgf]

same problem in .net 6

[SteffenSchwaiger]

Same problem in .NET 8.

Also, the workaround only works if TopMost = true is set on the child dialog after ShowDialog is called (e.g. inside the child form's Load event handler).

[xv]

Same problem in .NET 8.

Also, the workaround only works if TopMost = true is set on the child dialog after ShowDialog is called (e.g. inside the child form's Load event handler).

I came across this issue after upgrading to .NET 8 from Framework 4.8.

Setting the child form's TopMost after instantiating it but prior to calling ShowDialog() also works:

using var frmChild = new FrmChild();
frmChild.TopMost = TopMost;
frmChild.ShowDialog();

[Jaex]

We also migrated to .NET 9.0 from .NET Framework 4.8 and having same issue: https://jaex.getsharex.com/2025/06/q1Y7l3WOF73x.mp4

Even we set owner handle on ShowDialog method, still dialogs opening behind top most window. This was not the case with .NET Framework 4.8.

[Jaex]

My workaround:

public static DialogResult ShowDialogTopMost(this Form form, Form owner)
{
    if (owner != null && owner.TopMost)
    {
        form.TopMost = true;
    }

    return form.ShowDialog(owner);
}