it's not possible to move an undecorated window under a permanent panel at top of the screen, not even with ALT+Button1.
yes, that is by design, since it is undecorated, there is no way to tell what is 1) visible (could be shaped) and 2) what handle it uses.
I guess that rule could be loosen though
Yes, i guess so. I was just wondering because the top panel was the only one which showed this behaviour, whereas side and bottom panels didn't have that restriction.