Trekker Chair is redisgned the above photo is the old design

Below is the newer design. The critical difference is notice the upper segment is open at the top so excess mat can protrude up your back.

Compare with the earlier design notice the mat is constrained up the back.

The old design meant that the newer thicker mats, like the Neoair (and its successors, All Seasons, etc) were foced to double-up to fit inside the contrains and meant you had a too-thick rolling almost unworkable seat. The newer design you got a choice, you can double-up or protrude excess up the back, deciding how thick is the seat under and behind you.

There is still a design complaint. The stiffeners running parallel to the sitting person each side, the ones up the back go right to the edge of the top, the ones underneath stop shorter. Not sure why Thermarest did it this way, possibly so the stiffeners aren't digging into your legs??? However, its a missed opportunity because with the chair being open at one end, the excess mat could be under your legs doing more comfy goodness than up the back. Missed opportunity in the design there.....

I've got the lighter version the Compack Chair and tried with a Neoair Short, and it works well, the excess mat up the back is about my shoulder height and the chair is comfy, but obvious any cold tent floor is immediatel under my legs, and I got unused mat behind me.