Try it. Move your Steam window around. See how it snaps to the edges of your display(s)?
Why is Steam the only program (I know) that does that natively? I would love for ALL my Windows programs to do that (without installing 3rd party softwares/scripting).
EDIT : I’m on Win10.
I know this shortcut, and it resizes the window to half-screen. Got any shortcut that does the same, without resizing?
Check out Multitasking settings and see if any of those do what you are asking.
IIRC, Windows key and Up arrow will maximize it. Not aware of a way to snap a window to the top without it changing size. Would be pretty cool.
You might want to check out Windows PowerToys and specifically Fancy Zones. You can create your own areas to snap windows into. Still not quite what you’re looking for but might provide an alternative approach.