Leak between copper and braided supply line
I figured it out.  I'm used to always holding the backside steady with a second wrench when tightening anything, so I was afraid to try this here.  But after reading @brhans comments, I tried it, simply holding the brass sleeve with vice grips and tightening the connector a quarter turn and that did the trick.  I suppose the reason this piece doesn't have wrench flats is it's designed to be just finger tight, but in this case it required a little extra.  This connector has two rubber O-rings inside, so the threads do not make the seal.