More mouse enhancements for Mac OS X

The attached patch brings two more enhancements to mouse handling on Mac
OS X (Quartz):

  1. Currently, after launching an SDL application, SDL’s notion of the
    mouse position is stuck in the top left corner (0,0) until the first
    time the mouse is moved. That’s because the UpdateMouse() function isn’t
    implemented in the Quartz driver. This patch adds it.

  2. When grabbing input while the mouse cursor is hidden, the function
    CGAssociateMouseAndMouseCursorPosition(0) is called, which prevents the
    system’s notion of the mouse location from moving (and therefore leaving
    the SDL window) even when the mouse is moved. However, apparently the
    Wacom tablet driver (and maybe other special pointing device drivers)
    doesn’t care about that setting and still allows the mouse location to
    go outside of the window. Interestingly, the system cursor, which is
    made visible by the existing code in SDL in that case, does not follow
    the mouse location, but appears in the middle of the SDL window. The
    mouse location being outside of the window however means that mouse
    button events go to background applications (or the dock or whatever is
    there), which is very confusing to the user who sees no cursor outside
    of the SDL window.

I have not found any way of intercepting these events (and that’s
probably by design, as “normal” applications shouldn’t prevent the user
from bringing other applications’ windows to the front by clicking on
them). An idea would be placing a fully transparent, screen-filling
window in front of everything, but I fear that this might affect
rendering performance (by doing unnecessary compositing, using up
memory, or whatever).

The deluxe solution to the problem would be talking to the tablet
driver using AppleEvents to tell it to constrain its mapped area to the
window (see Wacom’s “TabletEventDemo” sample app,, but I think that
the bloat that solution would add to SDL would outweigh its usefulness.

What I did instead in my patch is reassociating mouse and cursor when
the mouse leaves the window while an invisible grab is in effect, and
restoring the grab when the window is entered. That way, the grab can
still be effectively broken by a tablet, but at least it’s obvious to
the user that it is broken. That change is minimal - it doesn’t affect
operation with a mouse (or a trackpad), and the code that it adds is not
executed on every PumpEvents() call, only when entering and leaving the

Unless there are any concerns about the patch, please apply. Feel free
to shorten the lengthy comment in SDL_QuartzEvents.m if you think it’s
too verbose.


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Name: SDL-quartz-mouse-stuff.patch

Christian Walther wrote:

The attached patch brings two more enhancements to mouse handling on Mac
OS X (Quartz):

These are in CVS now, thanks!