ui/cocoa.m: replace scrollingDeltaY with deltaY

The NSEvent class method scrollingDeltaY is available
for Mac OS 10.7 and newer. Since QEMU supports Mac OS
10.5 and up, we need to be using a method that is
available on these version of Mac OS X. The deltaY
method is a method that does almost the same thing as
scrollingDeltaY and is available on Mac OS 10.5 and
up. So we can replace scrollingDeltaY with deltaY.

We only check deltaY's value if it is not zero
because zero means that the scrolling increment was
sufficiently fine that it was only reported in scrollingDeltaY,
or that the scrolling was horizontal.
Signed-off-by: NJohn Arbuckle <programmingkidx@gmail.com>
Message-id: 20180709150235.7573-1-programmingkidx@gmail.com
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
[PMM: tweak commit message and comment a little]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

ui/cocoa.m

@@ -802,14 +802,20 @@ QemuCocoaView *cocoaView;
              * This is in-line with standard Mac OS X UI behaviour.
              */
 
+            /*
+             * When deltaY is zero, it means that this scrolling event was
+             * either horizontal, or so fine that it only appears in
+             * scrollingDeltaY. So we drop the event.
+             */
+            if ([event deltaY] != 0) {
             /* Determine if this is a scroll up or scroll down event */
-            buttons = ([event scrollingDeltaY] > 0) ?
-                INPUT_BUTTON_WHEEL_UP : INPUT_BUTTON_WHEEL_DOWN;
-            qemu_input_queue_btn(dcl->con, buttons, true);
-            qemu_input_event_sync();
-            qemu_input_queue_btn(dcl->con, buttons, false);
-            qemu_input_event_sync();
-
+                buttons = ([event deltaY] > 0) ?
+                    INPUT_BUTTON_WHEEL_UP : INPUT_BUTTON_WHEEL_DOWN;
+                qemu_input_queue_btn(dcl->con, buttons, true);
+                qemu_input_event_sync();
+                qemu_input_queue_btn(dcl->con, buttons, false);
+                qemu_input_event_sync();
+            }
             /*
              * Since deltaY also reports scroll wheel events we prevent mouse
              * movement code from executing.