Touch keyboard closed when touching a TextBox in PopupBox for the first time

[mgnslndh]

Observed behaviors

• Touching a TextBox works fine and will bring up the touch keyboard.
• Touching a TextBox inside a PopupBox for the first time opens up the touch keyboard but it is closed immediately. 💥
• Touching a TextBox inside a PopupBox for the a second time opens up the touch keyboard and it stays open.

Reproduction Reposititory

https://github.com/mgnslndh/MaterialDesignXamlTouch

[mgnslndh]

This behavior can be observed also for the default dialog style. Probably because it is also using PopupEx behind the scenes. The embedded dialog style works as expected. No PopupEx involved in that one.

[mgnslndh]

This issue is probably related to the original Popup control and I have found the same issue reported in the WPF forums with a workaround:
https://docs.microsoft.com/en-us/answers/questions/40137/wpf-soft-keyboard-shows-then-hides-in-popup-window.html

The workaround seems to work for Popup and I will try it on PopupBox as well.

[mgnslndh]

I could not get the workaround to work with PopupBox. For my scenario I ended up using the embedded style for dialogs instead. This does not use another window or adorner so the touch keyboard seems to work out of the box. This is not to say that the workaround cannot be applied to the PopupBox, just that I was not able to do it during the limited amount of time available.