If I set:
SkinManager.SkinFilter := [sfiInternal]; at design time, (so basically I turn OFF external)
>> adding a skin at InternalSkins panel is not more possible. (Because it's not listing anything at the Open-dialog).
Of course I can temporary turn back on now, since it has happened 5 minutes ago, but that will be a big problem, after a few month / years >> when I have forgotten I've turned off that option, hidden under a [-] sign…
So I think DESIGN time behavior should be always TRUE by default, when someone clicks the ADD button inside the manager panel.
(But only than! Disabling of listing externals should be still possible, except for the ADD button.)