@Gig : Exactly, not required but only to correct the side-effect induced by in_mouse -1. If you don't alt-tab, you don't need it
I'm glad it helped ^^. But I still wonder how the negative acceleration interferes with gameplay. If I get things correctly, with in_mouse -1, mouse input is handled by Windows, instead of SDL input, and negative accel is the major downside. I was also quite surprised that such setting worked because in other games where mouse lag is occured, using raw input solves the problem definitely.
In my case I have to set my DPI settings down to 400-600 dpi instead of 800 dpi on Win 8.0 & in_mouse 1 (all settings like fov, sensitivity, windows mouse setting set to default except for EPP, and using a Roccat Kone xtd mouse), to get close of the old mouse speed, but I feel there is "something that's not right" when I flick shoot. It is maybe due to "poor habits" in settings or style of gameplay but not sure.
Any input about in-game experience and/or aiming would be greatly appreciated
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EDIT 1 :Also noticing it is sometimes playable with a Logitech mouse (g500 with Gaming software suite installed), pollrate set to 125Hz, in_mouse 1, after some switching to fullscreen <> Windowed. But mouse can behave differently when opening the in-game menu.
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EDIT 2 :the old executable (Openarena-deprecated.exe with light blue logo, time stamp 2008-10-20) does not have the problem when using in_mouse 1, but clears any settings (make a backup of q3config in C:\Users\<username>\AppData\Roaming\OpenArena first!).
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MS finally acknowledges the issue, promises fix, no ETA
http://www.bit-tech.net/news/bits/2013/11/04/win81-mouse/