Fixed bug 2346 - Mac: mousewheel events have flipped horizontal scroll values

Alex Szpakowski

On my Mac OS X system (10.9.1), the SDL_MOUSEWHEEL event reports negative X values when my trackpad scrolls to the right, and positive X values when my trackpad scrolls to the left. This is backwards from what I'd expect, and I don't think it matches the Windows wheel events.

The vertical scroll values are what I'd expect though, and are consistent what gets reported on Windows (positive Y for scrolling up, negative Y for scrolling down.)

This is with "scroll direction: natural" disabled in the OS X trackpad settings (i.e. my scroll direction in non-SDL OS X programs matches what happens in Windows and Linux.)

I also tested with the horizontal scroll on a real mouse (Logitech G500 without custom drivers), and the horizontal scroll values in SDL are still flipped.

I "solved" the issue for myself by changing this line in the Cocoa_HandleMouseWheel function:

    float x = [event deltaX];

to this:

    float x = -[event deltaX];

I believe it should work fine with that change - I found something similar in another codebase while looking online for my issue - but I haven't tested on anything below Mac OS 10.8.

include/SDL_events.h

@@ -257,8 +257,8 @@ typedef struct SDL_MouseWheelEvent
     Uint32 timestamp;
     Uint32 windowID;    /**< The window with mouse focus, if any */
     Uint32 which;       /**< The mouse instance id, or SDL_TOUCH_MOUSEID */
-    Sint32 x;           /**< The amount scrolled horizontally */
-    Sint32 y;           /**< The amount scrolled vertically */
+    Sint32 x;           /**< The amount scrolled horizontally, positive to the right and negative to the left */
+    Sint32 y;           /**< The amount scrolled vertically, positive away from the user and negative toward the user */
 } SDL_MouseWheelEvent;
 
 /**

src/video/cocoa/SDL_cocoamouse.m

@@ -291,7 +291,7 @@ Cocoa_HandleMouseWheel(SDL_Window *window, NSEvent *event)
 {
     SDL_Mouse *mouse = SDL_GetMouse();
 
-    float x = [event deltaX];
+    float x = -[event deltaX];
     float y = [event deltaY];
 
     if (x > 0) {