/**
@page scrolling Scrolling

libinput supports three different types of scrolling methods: @ref
twofinger_scrolling, @ref edge_scrolling and @ref button_scrolling. Some
devices support multiple methods, though only one can be enabled at a time.
As a general overview:
- touchpad devices with physical buttons below the touchpad support edge and
  two-finger scrolling
- touchpad devices without physical buttons (@ref clickpad_softbuttons
  "clickpads") support two-finger scrolling only
- pointing sticks provide on-button scrolling by default
- mice and other pointing devices support on-button scrolling but it is not
  enabled by default

A device may differ from the above based on its capabilities. See
libinput_device_config_scroll_set_method() for documentation on how to
switch methods and libinput_device_config_scroll_get_methods() for
documentation on how to query a device for available scroll methods.

@section horizontal_scrolling Horizontal scrolling

Scroll movements provide vertical and horizontal directions, each
scroll event contains both directions where applicable, see
libinput_event_pointer_get_axis_value(). libinput does not provide separate
toggles to enable or disable horizontal scrolling. Instead, horizontal
scrolling is always enabled. This is intentional, libinput does not have
enough context to know when horizontal scrolling is appropriate for a given
widget. The task of filtering horizontal movements is up to the caller.

@section twofinger_scrolling Two-finger scrolling

The default on two-finger capable touchpads (almost all modern touchpads are
capable of detecting two fingers). Scrolling is triggered by two fingers
being placed on the surface of the touchpad, then moving those fingers
vertically or horizontally.

@image html twofinger-scrolling.svg "Vertical and horizontal two-finger scrolling"

For scrolling to trigger, a built-in distance threshold has to be met but once
engaged any movement will scroll. In other words, to start scrolling a
sufficiently large movement is required, once scrolling tiny amounts of
movements will translate into tiny scroll movements.
Scrolling in both directions at once is possible by meeting the required
distance thresholds to enable each direction separately.

Two-finger scrolling requires the touchpad to track both touch points with
reasonable precision. Unfortunately, some so-called "semi-mt" touchpads can
only track the bounding box of the two fingers rather than the actual
position of each finger. In addition, that bounding box usually suffers from
a low resolution, causing jumpy movement during two-finger scrolling.
libinput does not provide two-finger scrolling on those touchpads.

@section edge_scrolling Edge scrolling

On some touchpads, edge scrolling is available, triggered by moving a single
finger along the right edge (vertical scroll) or bottom edge (horizontal
scroll).

@image html edge-scrolling.svg "Vertical and horizontal edge scrolling"

Due to the layout of the edges, diagonal scrolling is not possible. The
behavior of edge scrolling using both edges at the same time is undefined.

Edge scrolling overlaps with @ref clickpad_softbuttons. A physical click on
a clickpad ends scrolling.

@section button_scrolling On-Button scrolling

On-button scrolling converts the motion of a device into scroll events while
a designated button is held down. For example, Lenovo devices provide a
<a href="http://en.wikipedia.org/wiki/Pointing_stick">pointing stick</a> that emulates
scroll events when the trackstick's middle mouse button is held down.

@note On-button scrolling is enabled by default for pointing sticks. This
prevents middle-button dragging; all motion events while the middle button is
down are converted to scroll events.

@image html button-scrolling.svg "Button scrolling"

The button may be changed with
libinput_device_config_scroll_set_button() but must be on the same device as
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
*/
