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

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

从链接的 MaterialDesignXamlTouch 复现开始，检查 issue 中描述的 PopupBox/TextBox 交互。复现首次触摸时的行为，然后验证触摸键盘在首次交互时仍保持打开，同时保留现有的第二次触摸行为。