The bug has been fixed here, no idea why it was happening but for some reason everything is back to normal here luckily
edit: 30 mins after this post:
the bug is different, just the osu logo and its menu is not rendering at all, cursor behavior is normal (it can be moved and it doesn't leave a permanent mark all over)
This bug was after I accidentally clicked "get more skins" which shut down osu! and gave me the osume.exe with the legacy skins, I started osu! directly from the updater because I don't need any of these old skins (why doesn't it simply open a browser window of the skins forum? ah well)
Edit2: this bug now happens every time when I start osu! after it asks me to enter my login and password (because of my policy to never remember any of my passwords on any machine of anything, email, and whatever too) osu! goes from full screen into windowed mode, to give me this dialog, and that's when the bug starts.