How often do you use the NSPanel?
I have not used NSPanels, except for the NSOpenPanel and NSSavePanel, in my programs.
So, I didn’t remember a littler different behaviour of NSPanel.
Recently I found out that an NSPanel doesn’t accept keyboard shortcuts if its style is “Utility“.
When your NSPanel doesn’t allow you to copy and paste text for NSTextFields located on the NSPanel, try checking if “utility” style is on. If so, remove it to enable copy & paste.