I am glad to hear you managed to achieve most of the desired functionality.
As I understand your last requirement would be to be able to start an extended selection from within a details RadGridView and continue with it in the parent RadGridView.
As the user holds the Shift key and presses the Down arrow key, there are two commands RadGridView executes internally. They are
1. "Move down"
2. "Extends selection to current unit".
I am afraid there is not a command available in RadGridViewCommands to manage an action going back to the parent RadGridView and continuing the selection from the item right after the current parent row. Still, I can suggest you a possible workaround. For the purpose you will need to define your own custom keyboard provider. That way you will be able to redefine the way RadGridView internally handles the operations. You can refer to our online documentation for further details and also find some sample code. As a side note, you can also check this GitHub example on CustomKeyboardCommandProvider. It demonstrates how to predefine the default KeyboardProvider.
I prepared a sample project to illustrate how you could achieve your goal. Basically I find the parent RadGridView, I set its CurrentCellInfo and then I define what RadGridViewCommands to be executed for it. You can find it attached.
Would you please check it and let me know how does it work for you?
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