Raw mouse input in Linux on SDL2 is not working properly though as far as I know. Could be wrong though, but it was one of the sticking points in the mailing list some time ago.

Yes, this can be made to work. Make sure you have the correct events selected.

A complete working example can be found here: OpenTK/Platform/X11/XI2MouseKeyboard.cs. This has been tested and is working on both X.org and XQuartz.

The source code is in C#, but most of it should be trivial to port to C or C++. Feel free to ask if you have any questions.

Edit: raw input has been pulled from SDL2 for some reason, which is rather unfortunate. You can only use regular input events (single mouse with pointer acceleration etc.)

Edit 2: do not use WarpMouse unless you really hate your users. That was necessary back in the core X11 days, due to the lack of relative motion events, but this is no longer necessary with XInput2. Do note that some devices, such as the VMware mouse driver, do not report relative motion events. Actual mice do not have this problem.

Edit: raw input has been pulled from SDL2 for some reason, which is rather unfortunate. You can only use regular input events (single mouse with pointer acceleration etc.)

It was never working properly in the first place. In fact I think only in OSX it ever worked properly.