Very little — roughly half the leaf width projecting, because the door folds at its centre hinge rather than swinging as one piece.

That is what makes it the standard answer in a narrow bathroom, where the distance between the shower and whatever faces it is the binding constraint.

It is worth measuring that facing obstruction rather than the room, since the thing that stops a door opening is usually a basin pedestal or a radiator rather than the wall opposite.

The hinge set halfway across the leaf, where the door breaks to fold. It carries the movement of both halves and takes the most cycles of any part of the door.

It is therefore the part that wears first, showing as a door that no longer folds cleanly or drops slightly out of line as it closes.

Better ranges make the fold hinge serviceable rather than swaged shut, which is worth checking, because a worn fold hinge on a sealed assembly means replacing the door.

It can, and it is the trade-off. The fold creates a vertical break halfway along the leaf, which is one more line to seal than a single-piece door has.

Manufacturers deal with it by running a seal strip down the fold and shaping the profiles so water is directed inward, but the joint remains the place to inspect on a door that is leaking.

Directing the showerhead away from the fold, which usually means mounting it on the opposite wall, removes most of the problem before it starts.

It has more moving parts and a break in the middle, so it is less rigid — but it is also lighter, and the loads it carries are correspondingly smaller.

In practice failures are wear rather than breakage: the fold hinge, the runner at the top, and the seals.

Where a door is going to be pulled on heavily, or used by someone who needs to steady themselves, a hinged leaf on solid hinges is a better choice than any folding door.

Most can, and it usually amounts to turning the door and moving the handle rather than buying a different product.

The instructions will state whether the top runner and the seals also need to be reversed, and skipping that step is why a reversed door sometimes leaks on the hinge side afterwards.

Confirming reversibility before ordering matters most on a replacement, where the opening hand is already fixed by everything else in the room.