Here is a commit from Brecht which could be a first step before having child windows that stay on top of the main window
https://developer.blender.org/rB4e2228525fc9bbcff58e315c5e3bbc193cd63a0b
It has to be tested to see how it works at the moment but I think it is a great improvement. Let’s try that 
YEAH … awesome! looking forward testing it
Unfortunately it doesn’t really say that the child windows will stay on top. It seems to be just a dupe without a top or status bars.
“Note these changes do not currently affect how the operating system treats the windows”
Yes indeed, but maybe it is planned, as it says that it does not “currently” affect how the operating system treats the windows 
I have hope haha
Actually, you are right. It look like the sentence holds hope