MaterialDesignDataGridTextColumnPopupEditingStyle: Mouse and focus issues

When you double click on a cell which has EditingElementStyle="{StaticResource MaterialDesignDataGridTextColumnPopupEditingStyle}" defined, the popup appears but there are a few issues:

1. You can't move the cursor using the mouse and can't select text with the mouse (although right clicking brings up the context menu correctly).
2. If the cell editing popup is open and you change focus to another app, the popup remains on top of the other application.
3. (Minor/opinion) The cursor's position is at the beginning of the TextBox when the popup editor opens (I think this would typically be at the end?)

To reproduce:

• Open the WPF demo
• Selecting the "Grids" page using the menu
• Double click on a cell in the Name or Description columns to start editing
• Try using the left mouse button to move the cursor or select text inside the popup
• Switch to another application while the cell editor is open

Here's a screenshot of the popup over another window.

从 WPF 示例的 Grids 页面以及 Name 和 Description 列使用的 MaterialDesignDataGridTextColumnPopupEditingStyle 开始。通过双击复现弹出窗口的行为，然后跟踪弹出编辑器对鼠标选择、应用程序焦点和初始光标位置的处理。完成的标准是鼠标编辑正常工作，弹出窗口不会停留在另一个应用程序之上，并且光标行为得到处理。