It’s the primary screen, i guess. it’s the wide screen with 1200px height.
I dont know if the in-game window not save the last position. We only know that windows keeps resetting. I think it saves it but is resetted at next restore.
User drag the window in a certain position.
ALT+G (for example) so hide and restore the window
The window resets
What i’ve seen using developer tools is the window was 1200px height while the normal size is 800px but most part of the window is transparent so anyone knows the real size without inspecting. Other users is reporting the same thing.
Going a bit deep, the window resets its position on the vertical axis:
User drag to 200, 100. Resets at 200, 0 (in this specific case when the window have height = screen height)