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 and (because of my policy to never any of my s 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.