Specifically, I am interested in perpendicularity. I have a plastic lift and slide unit where the vertical frame is not at right angles. When the sash is open, this is clearly visible. At the bottom the glass disappears behind the fixed element, at the top I have a view of almost 2 cm.
Is this a defect? Or should I put up with it?
Yes, it looks silly, but if everything closes tightly, it's probably a minor defect.
I mean, 2 cm by 2 m (which is probably more, isn't it?) is only a 1% deflection then. Annoying, but I don't think there's anything you can do about it.
Is the window crooked or the frame?
2 or 3 cm with the window closed is a giant difference.
This looks very weird. I have not seen anything like this before. Still can't imagine what could cause this.