Popup is closed while selecting date from the DatePicker

* Have you experienced this same bug with .NET Framework?:
4.6 Framework

**Problem description:**

I have created the WPF project. I have added the date picker inside the pop up and added combo box in another popup. I have set popup.staysopen to false for both popup. Now Im clicking the date picker and calendar is opened in outside the window. When I click the date from calendar which is in outside the region(As like below image), the popup closed.


Meanwhile this issue not happens in the combo box.

**Expected behavior:**

Popup should not closed when focus is in the date picker.

**Minimal repro:**
Run the sample
Click the Date Picker Button
Select the date as I mentioned in the problem description
Pop will closed.

Please find sample below
[Sample.zip](https://github.com/dotnet/winforms/files/5994466/Sample.zip)