The silver sector is the raised floor. Upon walking over the green action line in the centre, the silver sector lowers to the height of the floor around it. (Both the main brown sector and the gray 'monster closet' areas have the same floor height)

Now here is my question. The silver floor is currently changing to the dark gray flat, but I want it to change to the brown flat. What is the mechanism that the Doom engine uses to determine which adjacent flat the lowering sector changes to?

I have tried different combinations of flipping the two sided linedefs connecting the silver sector to the sectors either side of it. (Frontside facing brown, backside facing gray, etc,etc), and I have tried creating dummy sectors. (You can see the silver square surrounded by the bigger brown square in the bottom right.)

In a worst case scenario, I can alter the floor height so that the gray floor sits one pixel higher, and that solves the problem. But I would like to keep the floor heights the same if I can.

And it would be better to understand how it actually works.

(ZDoom shot with gray floor raised by one pixel, missing lower texture; much less noticeable than having a one unit high texture there.)

Thanks guys.