So about the clipping on Longcoats/Robes

[Nuinn]

But...this is a top! It's a jacket! I mean granted some of the bottoms that clip include the spring skirt, but Robes and Jackets?!

Oh, right.... I forgot to mention I wear just as little on the top as well

Anyways, I feel for you. Joke aside, it's why I'm very wary of any gear that looks bulky/long/too elaborate. Clipping makes me twitch so bad I snap my neck each time :/
Not sure what could be done in the case of this coat. I've considered getting it cause I like the design but once I realized that horrible clipping would occur, you couldn't even gift it to me for 0 Gil.

Most robes (which are chest pieces as well) don't have clipping, or have it in minimal amounts. Presumably cause the robes have been designed (rigged) differently than this coat, which seems to be just a "jacket type" chest piece on which they slapped a sheet of polygons at the bottom, without rigging it to have the physics the lower portions of robes have.