Re: [Qemu-devel] [PATCH] ui/cocoa.m: replace scrollingDeltaY with deltaY

[Peter Maydell]

On 3 July 2018 at 05:13, John Arbuckle <address@hidden> wrote:
> 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 the same thing as scrollingDeltaY and is available on Mac OS 10.5 
> and up. So we simply replace scrollingDeltaY with deltaY.

I think that this change will result in scroll events from
high-scrolling-precision devices (eg magic trackpad)
sometimes giving the wrong kind of event.

In particular, this comment from a random Eclipse bug
report has a good description of what happens:
 https://bugs.eclipse.org/bugs/show_bug.cgi?id=220175#c13

For high-precision scrolling devices, the app gets events
which:
 * have hasPreciseScrollingDeltas true
 * have scrollingDeltaY something non-zero
 * may have deltaY be zero

So if we use
 ([event deltaY] > 0) ? INPUT_BUTTON_WHEEL_UP : INPUT_BUTTON_WHEEL_DOWN

then for a slow upward scroll on a high precision device
some (most) of our up-scroll events will appear with deltaY == 0,
which this ?: will interpret as a WHEEL_DOWN event.

Two choices I can see here:

(1) make the code ignore ScrollWheel events with a deltaY of zero
(ie don't inject any QEMU button wheel event for them). Would
need to test on a 10.7+ system with a suitable precise-scrolling
device that this does actually still allow scrolling.

(2) use scrollingDeltaY on OSX versions that have it, and
only use deltaY on the older versions.

PS: could you line-break your commit messages, please?
They seem to be coming out as all one very long line.
Recommendation is hard linebreaks at about 70 chars per line.

thanks
-- PMM