I was reporting it at least 3 times already in last 7-8 months, but I think it never was addressed, nor confirmed, so time for 4-th report 
I have no idea how browser (Firefox) or it's addons can affect the Raw Input, or conflict with it, making cursor in osu! to move few times faster, but it certainly does. Last report, most detailed, is here.
I am not sure if it's either this flash player game or this Unity based game, but there is a high probability that it can happen for both of them independently. At least I experienced it with both of them, so maybe it's browsers fault?
Anyway, I was able to reproduce it even today. It needs a little time, sometimes like 2-3 minutes, sometimes 10-15 minutes or more, but I think you should be able to reproduce it too.
What I did? I opened these games, mostly the first flash player one, played it like 10 minutes, leaving it to do nothing in meantime, then just left it running. After few more minutes, my cursor in osu! went crazy. Turning off Raw Input fixes it, turning it on makes it broken again. Things like Fullscreen don't matter, after turning Fullscreen off it's still broken.
I started switching stream releases and it was bugged on every single one, including fallback. Cursor is crazy fast only with Raw Input on.
Then, I closed both games and cursor went back to normal. I opened the second game (Unity one) and it become bugged again. I closed the game again - cursor back to normal. I opened the game again - cursor is still normal(?).
Sometimes I had to close the browser to fix it. I once needed to restart my PC, but it was before I concluded that the Firefox is the guilty one, so not sure, if I closed it back then.
As you see, the problem is very weird, but it's present from many, many months, persisting through different releases and updates. I think that the Firefox or it's addons are probably in fault here (Flash Player?). I think that things like youtube are okay for me, what bugs the cursor for me are flash player games... but... how they can possibly affect Raw Input? I have no idea why it happens and how exactly Raw Input works, but it certainly does happens
Well, from some time I either close or restart browser anyway, to avoid osu! lagging on my potato laptop However, due to the very strange nature of this problem, I think that it should be addressed and tried to be reproduced. Try running and playing these games for a few minutes, leave them opened in the browser, maybe even restart osu! after some time, try playing the games again... it should appear after some time. Or maybe, with the knowledge about how Raw Input works, you can find an issue without doing that?
Anyway, I wish you luck! I love osu! and want it to work as best as it can, so I hope that whatever it is, you will find the issue, sooner or later, and will be able to fix it =)
Feel free to ask for more information and check the post I gave link to above^^.
I have no idea how browser (Firefox) or it's addons can affect the Raw Input, or conflict with it, making cursor in osu! to move few times faster, but it certainly does. Last report, most detailed, is here.
I am not sure if it's either this flash player game or this Unity based game, but there is a high probability that it can happen for both of them independently. At least I experienced it with both of them, so maybe it's browsers fault?
Anyway, I was able to reproduce it even today. It needs a little time, sometimes like 2-3 minutes, sometimes 10-15 minutes or more, but I think you should be able to reproduce it too.
What I did? I opened these games, mostly the first flash player one, played it like 10 minutes, leaving it to do nothing in meantime, then just left it running. After few more minutes, my cursor in osu! went crazy. Turning off Raw Input fixes it, turning it on makes it broken again. Things like Fullscreen don't matter, after turning Fullscreen off it's still broken.
I started switching stream releases and it was bugged on every single one, including fallback. Cursor is crazy fast only with Raw Input on.
Then, I closed both games and cursor went back to normal. I opened the second game (Unity one) and it become bugged again. I closed the game again - cursor back to normal. I opened the game again - cursor is still normal(?).
Sometimes I had to close the browser to fix it. I once needed to restart my PC, but it was before I concluded that the Firefox is the guilty one, so not sure, if I closed it back then.
As you see, the problem is very weird, but it's present from many, many months, persisting through different releases and updates. I think that the Firefox or it's addons are probably in fault here (Flash Player?). I think that things like youtube are okay for me, what bugs the cursor for me are flash player games... but... how they can possibly affect Raw Input? I have no idea why it happens and how exactly Raw Input works, but it certainly does happens
Well, from some time I either close or restart browser anyway, to avoid osu! lagging on my potato laptop
However, due to the very strange nature of this problem, I think that it should be addressed and tried to be reproduced. Try running and playing these games for a few minutes, leave them opened in the browser, maybe even restart osu! after some time, try playing the games again... it should appear after some time. Or maybe, with the knowledge about how Raw Input works, you can find an issue without doing that?Anyway, I wish you luck! I love osu! and want it to work as best as it can, so I hope that whatever it is, you will find the issue, sooner or later, and will be able to fix it =)
Feel free to ask for more information and check the post I gave link to above^^.