Robert Turner
SDL_windowsevents.c contains code to retrieve the x and y coordinate for a requested hit test. It does this as follows:
POINT winpoint = { (int) LOWORD(lParam), (int) HIWORD(lParam) };
LOWORD(lParam) does not correctly mask off high bits that are set if the point is on a second (or third, etc.) monitor. This effectively offsets the x-coordinate by a large value.
MSDN documentation suggests that LOWORD() and HIWORD() are the wrong macros for the task, instead suggesting we should be doing something like the following:
POINT winpoint = { GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam) };
Testing this change on my Windows 10 machine with 2 monitors gives the correct results.