The early TNG jumpsuits closed up the front via an invisible zipper.
With one exception, the jumpsuits were always fastened closed in the show – the sole exception being when space-drunk Beverly made a pass at Captain Picard. (“Of course, we haven’t time for that sort of thing …”)
Between takes, though, the actors would sometimes unzip their uniform jumpsuits a bit.
Zipper Seam Allowances
The center front/zipper seam allowances tended to be wider than those on the rest of the jumpsuit.
On the screen-used Worf TNG jumpsuit I examined, the front allowances were about ⅝”.
On the screen-used Dr. Crusher jumpsuit I examined they were about ¾”.
On the screen-used Tasha Yar jumpsuit I examined they were closer to ⅞”.
And on the Worf jumpsuit, these front allowances were actually tacked open by hand at the waist.
The top of the zipper was actually left hanging free, as to fully conceal the pull tab; the area above the zipper was fastened closed via hook-and-eye closures.
At the upper front, the yoke allowances were simply turned under and tacked down on the underside.
Curiously, on the Tasha Yar and Dr. Crusher uniforms I studied, there were two hook-and-eye closures above the zipper, but on the Worf uniform there were three.
Ideally these hook-and-eye closures tightly fastened the upper front area, but they can pull apart a little; thanks to the incredible quality of the show in remastered HD, the closures were actually visible sometimes.
In theory, this system is brilliant and should produce a completely hidden closure.
However, it does seem to have taken the costume department a few episodes to refine the process; many of the closures on the initial uniforms were somewhat … unsightly (at least by later standards).
The costumers began improving the process almost immediately; the unfortunate gaps at the top were tightened considerably … but not completely (yet).
While a major improvement, many of these uniforms still had a slight sliver of black at the upper front.
Another round of touch-ups had this upper front gap tightened even more, although this sometimes resulted in a slight “divot” where the closure tension was transitioned from the zipper to the hooks-and-eyes.
After a few episodes of practice and revision, the costume department had improved the process as to typically produce closures that were entirely hidden, literally appearing “seamless.”