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doc: add a blurb about scroll sources to the scrolling docs

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Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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Peter Hutterer 2015-05-28 08:53:00 +10:00
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doc/scrolling.dox
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@ -63,4 +63,23 @@ the motion events. Cross-device scrolling is not supported but
for one exception: libinput's @ref t440_support enables the use of the middle
button for button scrolling (even when the touchpad is disabled).
@section scroll_sources Scroll sources
libinput provides a pointer axis *source* for each scroll event. The
source can be obtained with the libinput_event_pointer_get_axis_source()
function and is one of **wheel**, **finger**, or **continuous**. The source
information lets a caller decide when to implement kinetic scrolling.
Usually, a caller will process events of source wheel as they come in.
For events of source finger a caller should calculate the velocity of the
scroll motion and upon finger release start a kinetic scrolling motion (i.e.
continue executing a scroll according to some friction factor).
libinput expects the caller to be in charge of widget handling, the source
information is thus enough to provide kinetic scrolling on a per-widget
basis. A caller should cancel kinetic scrolling when the pointer leaves the
current widget or when a key is pressed.
See the libinput_event_pointer_get_axis_source() for details on the
behavior of each scroll source.
See also http://who-t.blogspot.com.au/2015/03/libinput-scroll-sources.html
*/