libinput supports three different types of scrolling methods: Two-finger scrolling, Edge scrolling and On-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 (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.
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.
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.
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.
When a scroll gesture remains close to perfectly straight, it will be held to exact 90-degree angles; but if the gesture moves diagonally, it is free to scroll in any direction.
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.
On some touchpads, edge scrolling is available, triggered by moving a single finger along the right edge (vertical scroll) or bottom edge (horizontal scroll).
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 Clickpad software button behavior. A physical click on a clickpad ends scrolling.
Scroll sources are deprecated with libinput 1.19. The scroll source is now encoded in the event type.
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
Natural scrolling vs. traditional scrolling
Natural scrolling is the term (probably) coined by Apple for matching the motion of the scroll device with the direction of the content.
In traditional scrolling, moving the wheel down causes the scroll bar indicators to move down and the content to move up. In natural scrolling, moving the wheel down causes the content to move down and the scroll bar indicators to move up. This method of scrolling matches the interaction with content on touch screens where a movement down also moves the content down.
libinput supports natural scrolling for all its scroll methods; it can be enabled with the libinput_device_config_scroll_set_natural_scroll_enabled() function.