Matthew
Its possible to set SDL_CaptureMouse() so you continue receiving mouse input while the mouse is outside your window. This works however There is then a gap where no messages send, which is when the mouse is hovering the title bar and the window edges.
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Simple DirectMedia Layer
(SDL)
Version 2.0
---
http://www.libsdl.org/
Simple DirectMedia Layer is a cross-platform development library designed
to provide low level access to audio, keyboard, mouse, joystick, and graphics
hardware via OpenGL and Direct3D. It is used by video playback software,
emulators, and popular games including Valve's award winning catalog
and many Humble Bundle games.
More extensive documentation is available in the docs directory, starting
with README.md
Enjoy!
Sam Lantinga (slouken@libsdl.org)