When they first leave the ship.... in San Francisco, they walk off/down the stairs/lift of the ship... we see the stair rails But the ship is cloaked!
The cloaking field seems to extend around the normal extent of the ship. When objects protrude from this silhouette, they become visible. This is explicitly confirmed in the movie's novelisation:
For a minute Javy considered jumping out of the truck, but Ben had it going nearly fifty. Javy tried to see behind them in the side mirror, but the light and the ramp had vanished, and he could make out only shadows.
Jim led the way out of the Bounty and signaled for the ramp to withdraw. It disappeared into the cloaking field. The hatch closed, cutting off the interior light. Star Trek IV : Novelisation
We see another example of this when Scotty sticks the upper half of his body outside the field when he's manhandling the plexiglass walls into place:
Gillian gasped. The man hung unsupported in the air. But from the waist down, he did not even exist. It was as if he were standing within a structure that could not be seen and that could conceal him as well. An invisible structure ... "