LOL. I accept that I am open to mistakes, especially as age creeps in on me.I thought you used spacers all around ,both top,bottom and sides to prevent just that
I may be wrong,have been many times.
I do believe this is all more important in solid wood.
I have built all pine,all maple,maple and poplar and maple and mdf doors and did the same with all
Seemed to work,the pine are the oldest, now being replaced with the maple/ mdf,not because of issues with the doors but she wants a updated look
Sure everyone knows what that is
(Your comments on space balls) That's how I feel about it. The compression layer (say space balls are used) keep the panel centered, allow for an expansion joint, and keep the the door from rattling. It is modern, and currently how we deal with that. This is the currently accepted train of thought these days.
Personally, I try to stay away from MDF. It is great for making inexpensive paint-outs. I happen to prefer the look of wood grain. MDF is hard on my eyes, lungs and tooling. I use goggles and respirators when dealing with it... but it never quite seems to be good enough for my aging body.