Bug explanation

I clicked the Add fruit button on the DemoApp's dialogs tab Sample1 repeatedly. However, sometimes the focus is not set on the textbox of the Popup Dialog.
It appears irregularly. Sometimes it appears after repeating dozens of times, and sometimes it appears from the beginning.

It appears in DemoApp, Sourcecode, and nuget package of 4.6.1 Release.

Left is not focused. right is focused.

Version

4.6.1