libinput/doc/tapping.dox

/**
@page tapping Tap-to-click behaviour

"Tapping" or "tap-to-click" is the name given to the behavior where a short
finger touch down/up sequence maps into a button click. This is most
commonly used on touchpads, but may be available on other devices.

libinput implements tapping for one, two, and three fingers, where supported
by the hardware, and maps those taps into a left, right, and middle button
click, respectively. Not all devices support three fingers, libinput will
support tapping up to whatever is supported by the hardware. libinput does
not support four-finger taps or any tapping with more than four fingers,
even though some hardware can distinguish between that many fingers.

@section tapping_default Tap-to-click default setting

Tapping is **disabled** by default on most devices, see [this
commit](http://cgit.freedesktop.org/wayland/libinput/commit/?id=2219c12c3aa45b80f235e761e87c17fb9ec70eae)
because:
- if you don't know that tapping is a thing (or enabled by default), you get
  spurious button events that make the desktop feel buggy.
- if you do know what tapping is and you want it, you usually know where to
  enable it, or at least you can search for it.

Tapping is **enabled** by default on devices where tapping is the only
method to trigger button clicks. This includes devices without physical
buttons such as touch-capable graphics tablets.

Tapping can be enabled/disabled on a per-device basis. See
libinput_device_config_tap_set_enabled() for details.

@section tapndrag Tap-and-drag 

libinput also supports "tap-and-drag" where a tap immediately followed by a
finger down and that finger being held down emulates a button press. Moving
the finger around can thus drag the selected item on the screen.
Tap-and-drag is optional and can be enabled or disabled with
libinput_device_config_tap_set_drag_enabled(). Most devices have
tap-and-drag enabled by default.

Also optional is a feature called "drag lock". With drag lock disabled, lifting
the finger will stop any drag process. When enabled, libinput will ignore a
finger up event during a drag process, provided the finger is set down again
within a implementation-specific timeout. Drag lock can be enabled and
disabled with libinput_device_config_tap_set_drag_lock_enabled().
Note that drag lock only applies if tap-and-drag is be enabled.

@image html tap-n-drag.svg "Tap-and-drag process"

The above diagram explains the process, a tap (a) followed by a finger held
down (b) starts the drag process and logically holds the left mouse button
down. A movement of the finger (c) will drag the selected item until the
finger is released (e). If needed and drag lock is enabled, the finger's
position can be reset by lifting and quickly setting it down again on the
touchpad (d). This will be interpreted as continuing move and is especially
useful on small touchpads or with slow pointer acceleration.
If drag lock is enabled, the release of the mouse buttons after the finger
release (e) is triggered by a timeout. To release the button immediately,
simply tap again (f).

If two fingers are supported by the hardware, a second finger can be used to
drag while the first is held in-place.

@section tap_constraints Constraints while tapping

A couple of constraints apply to the contact to be converted into a press, the most common ones are:
- the touch down and touch up must happen within an implementation-defined timeout
- if a finger moves more than an implementation-defined distance while in contact, it's not a tap
- tapping within @ref clickpad_softbuttons "clickpad software buttons" may not trigger an event
- a tap not meeting required pressure thresholds can be ignored as accidental touch
- a tap exceeding certain pressure thresholds can be ignored (see @ref
  palm_detection)
- a tap on the edges of the touchpad can usually be ignored (see @ref
  palm_detection)

*/