[SOLVED] How to create a scroll wheel event that scrolls diagonally?

How to create a scroll wheel event that scrolls diagonally?

The docs are really stingy regarding this but from the info I gathered online, the wheelCount should be 1 for Y-only, 2 for Y-X, 3 for Y-X-Z.

To my understanding setting wheelCount = 2 inside CGEventCreateScrollWheelEvent should scroll both x and y by 10 pixels but the next chunk of code scrolls, in some apps (Chrome, Safari) only 10 pixels up while in other apps (Preview, Firefox) scrolls both axis:

    CGEventRef scroll = CGEventCreateScrollWheelEvent(NULL,
                                                  kCGScrollEventUnitPixel,
                                                  (uint32_t)2,(int32_t)10,
                                                  (int32_t)10);
    CGEventPost(kCGHIDEventTap, scroll);

    // Verify deltas

    int64_t deltaAxis1 = CGEventGetIntegerValueField(scroll, kCGScrollWheelEventPointDeltaAxis1);
    int64_t deltaAxis2 = CGEventGetIntegerValueField(scroll, kCGScrollWheelEventPointDeltaAxis2);

    NSLog(@"kCGScrollWheelEventPointDeltaAxis1: %lld",deltaAxis1); //kCGScrollWheelEventPointDeltaAxis1: 10
    NSLog(@"kCGScrollWheelEventPointDeltaAxis2: %lld",deltaAxis2); //kCGScrollWheelEventPointDeltaAxis2: 10

    CFRelease(scroll);

Link to docs

Update: After some digging, it seems that some apps don't know how to interpret multi wheel scrolling and scrolls only x or y.

This can be easily reproduced using the above code or a Macbook trackpad:

Open a zoomed in picture in Preview and in Safari. Swiping 2 fingers on a trackpad diagonally while in Preview will create a nice smooth x&y scroll. Doing the same inside Safari or Chrome will result only X or Y being scrolled and will feel broken in general.

I've found a Chrome bug related to this that was closed with "Won't fix". Can't understand why.

Any better explanation on why this is happening would be great.

Thanks

Solution

usesPredominantAxisScrolling describes the phenomenon:

Some content is scrollable in both the horizontal and vertical axes, but is predominantly scrolled one axis at a time. Other content (such as a drawing canvas) should scroll freely in both axes.

Traditionally this is not an issue with scroll wheels since they can only scroll in one direction at a time. With scroll balls and touch surfaces, it becomes more difficult to determine the user's intention.

This property helps a scroll view determine the user's intention by specifying if there is a predominant scrolling axis for content.

In a test app with a WebView, usesPredominantAxisScrolling of the scroll view is YES. Setting it to NO fixes the problem.