I'm not sure how much this applies to dwm, but in xmonad, there are 2 types of windows: floating windows, and sunk windows. Floating windows can be moved around and resized using the mouse. Sunk windows are arranged by the window manager rather than the user. So if you sink osu!, when it tries to move the window manager says "No, you go over there" and the window stays in the same spot.
And when I say "virtual desktop" I don't mean something like qemu or virtualbox. In winecfg, there's an option under the Graphics tab to create a windows desktop to run the wine program on rather than letting it use your Linux desktop. This desktop acts like a window in your Linux desktop with Windows windows inside it. If you run osu! in fullscreen there's only 1 window, so it looks normal.
And when I say "virtual desktop" I don't mean something like qemu or virtualbox. In winecfg, there's an option under the Graphics tab to create a windows desktop to run the wine program on rather than letting it use your Linux desktop. This desktop acts like a window in your Linux desktop with Windows windows inside it. If you run osu! in fullscreen there's only 1 window, so it looks normal.