The window sash is warped floor-to-ceiling, the door scrapes on the floor

Bonded with MS Polymer. You could hang a car on the bond... then it’s cheaper to replace it than to repair.

What I found online says something different. According to that, plastic frames without glass inserts are quite unstable, not very stiff, and are only stabilized by the glass insert. If it is not properly "blocked," the panel can sag towards the end, just as apparently (hopefully) happened with us. I’m not allowed to link anything here, so just a diagram:

Could be true. MS Polymer sticks like hell, but is tough and elastic.