Just starting to put my bike together. The groupset is XT 12speed have screwed the rear mech on the way that looks least wrong, but it does this:

flap flap

Am I being a numpty or should there be some kind of spring tension holding my mech in place?

Thats fine. Carry on.

Once the chain is in it’ll add the tension.

Yeah, that’s you want, it’s the range in motion of the mech untensioned, the good thing about 12sp is they come with the mark on the back of the hanger (right at the bottom) that allows you to adjust the b screw properly, saves a bit of faffing!

Yep, it won’t do that once you’ve finished setting it up.

Thanks everyone!