1op layers over infill, over which new perimeter walls will be printed, are extended until they reach the next infill wall (so they have something to stick to and aren't left hanging in thin air). This could be a checkable option right next to the top layer count option.
Imagine two cubes stacked, the lower one being much larger than the top one. The top cube is centered over the bottom cube. The top cube is printed on top of the top layers of the bottom cube, and under that is the infill of the bottom cube. The problem is that the perimeters of the top cube probably don't match up to the infill walls below it, which means the "top" layers of the bottom cube will just stop under the outline of the top cube, in thin air, and won't be well supported. In all likelihood additional top layers will have to be added just to ensure the lower cube's top layers can provide a decent support for the perimeter walls of the top cube. If the top layers of the bottom cube were extended until they hit the next infill wall, however, then they would be well supported and form a decent foundation for the perimeter walls of the feature printed over them.
This feature would benefit any model that has top layers over infill, and then another feature sticking up over those top layers somewhere on the model. This feature request would save printing time and filament over simply adding more top layers, or slicing time by avoiding having to use extra processes right at that top layer just to provide enough infill support for the top layers to ensure they can be a good support for the perimeters of the feature that sticks up from those top layers.