Non-active PanelStack2 panels should not be able to have focus within, even if rendered

[evansjohnson]

Steps to reproduce

• Ensure "Render active panel only" is toggled off
• Open a few panels
• Use keyboard nav with shift+tab to go back a few elements, and notice non-visible elements can receive focus and be interacted with

Actual behavior

Able to keyboard nav to non-visible items, user loses visual indication of what element is focused.

Expected behavior

Should not be able to focus and interact with non-visible panels.

Possible solution

Add focus relocating elements similar to what we do to trap focus in a Dialog to move focus from the first visible and focusable PanelStack2 item to the last focusable element before the PanelStack2