touchpad: add a middle button software area

Middle button interaction is most commonly to paste and it is a single-event
interaction (button press). We provided middle button in software button mode
by emulating it with a two-finger press with L+R down at the same time. This
is also what many touchpads are spectacularly bad at, it is very common to
detect the physical button down event before the second finger registers,
resulting in left or right clicks where a middle button should be triggered.

Unless the fingers are resting on the touchpad for at least one scanout, the
success rate for middle button emulation is only at 70% or so.

This patch adds a 25%-width middle button area between the left and the right
software button, everything else stays the same. To avoid immediate breakage,
the middle button emulation remains but may be removed in the future.
The doc is updated to only refer to the middle button area now.

https://bugs.freedesktop.org/show_bug.cgi?id=94755

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/doc/clickpad-softbuttons.dox b/doc/clickpad-softbuttons.dox
index a4f2e444..cf4c8c9e 100644
--- a/doc/clickpad-softbuttons.dox
+++ b/doc/clickpad-softbuttons.dox
@@ -20,16 +20,20 @@ generated by libinput and passed to the caller in response to a click.
 @section software_buttons Software button areas
 
 On most clickpads, this is the default behavior. The bottom of the touchpad
-is split in the middle to generate left or right button events on click. The
-height of the button area depends on the hardware but is usually around
-10mm.
+is split into three distinct areas generate left, middle or right button
+events on click. The height of the button area depends on the hardware but
+is usually around 10mm.
 
 Left, right and middle button events can be triggered as follows:
 - if a finger is in the main area or the left button area, a click generates
   left button events.
 - if a finger is in the right area, a click generates right button events.
-- if there is a finger in both the left and right button area, a click
-  generates middle button events.
+- if a finger is in the middle area, a click generates middle button events.
+
+The middle button is always centered on the touchpad and smaller in size
+than the left or right button. The actual size is device-dependent though as
+many touchpads do not have visible markings for the middle button the exact
+location of the button is not visibly obvious.
 
 @image html software-buttons.svg "Left, right and middle-button click with software button areas"