I think you may be right about window settings being persisted independently of my property setting. But - I tested now, and it does not seem to work that way. I open the filler, make some changes in the form, tried closing it, and submitting it. It does not rpersist screen nor window size. It is just very odd.
I have 900 px of excess whitespace all of the time, and would love to manipulate the behaviour to get rid of it.. I don't think center-aligning would fix the problem, would only get 450 px per side instead. It just looks very unprofessional with all that excess whitespace. Not to mention, the people using it may have something up on their screens they are using simultaneously. Ideally a window should not block all other contents in default restored state, only maximised stated.
Thank you for your suggestion, got any more..?