I've been noticing this for several weeks, ever since I installed Windows 10 (Currently build 10122, haven't tried osu! on the latest)
When playing in windowed mode (any resolution) while using the Beta or Cutting Edge streams, the game canvas seems offset from the top of the window (interestingly, by the height of the window title bar). Seems to also be slightly offset from the left edge.
Doesn't happen in fullscreen or borderless windowed, and doesn't happen in the stable stream when using either DirectX or OpenGL.
Screenshots:
Stable:
http://i.imgur.com/4PeG4k2.pngCutting Edge:
http://i.imgur.com/XTizBRt.jpgEdit: I should add that this could be a problem on Microsoft's end. There was a sort of similar issue in a previous W10 build where maximized windows (notably Chrome/Firefox) would have their tops disappear out of the screen area. Seems MS is having some trouble getting window borders/sizes right, so there's a good chance this is a generic issue and will be fixed (if it's not already) in a later build.
Edit 2: After speaking a bit with
@kevlinmannen on twitter, it seems that's happened. He's running the latest Windows build, and can't reproduce the problem.
Might be worth disregarding this whole report.