evdev: use distance triggers to start scrolling

The previous code used delta/event as scroll trigger which roughly translates
to speed, but depends on the sampling rate of the device.

For slow two-finger motion, a user may move the height of the touchpad without
ever triggering scrolling. Change the _initial_ trigger to a cumulative
trigger, i.e. once the user moved past the threshold distance, scrolling
starts regardless of the speed.

Once scrolling is engaged, the original trigger of threshold/event is
required to engange the second scroll direction.

Note that except for really slow movements, it's very easy to engage both
scroll directions on a touchpad. This is intentional, libinput does not have
enough semantic knowledge to know if horizontal scrolling is needed. So we
provide some direction locking but not much, it's up to the
client/toolkit/widget to decide if both scroll directions should be handled.
Add a comment to clarify that in the public doc.

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

1 files changed, 45 insertions, 0 deletions

diff --git a/test/touchpad.c b/test/touchpad.c
index f9031dd9..2b79aafe 100644
--- a/test/touchpad.c
+++ b/test/touchpad.c
@@ -1455,6 +1455,50 @@ START_TEST(touchpad_2fg_scroll)
 }
 END_TEST
 
+START_TEST(touchpad_2fg_scroll_slow_distance)
+{
+	struct litest_device *dev = litest_current_device();
+	struct libinput *li = dev->libinput;
+	struct libinput_event *event;
+	struct libinput_event_pointer *ptrev;
+
+	litest_drain_events(li);
+
+	litest_touch_down(dev, 0, 20, 30);
+	litest_touch_down(dev, 1, 40, 30);
+	litest_touch_move_to(dev, 0, 20, 30, 20, 50, 60, 10);
+	litest_touch_move_to(dev, 1, 40, 30, 40, 50, 60, 10);
+	litest_touch_up(dev, 1);
+	litest_touch_up(dev, 0);
+	libinput_dispatch(li);
+
+	event = libinput_get_event(li);
+	ck_assert_notnull(event);
+
+	/* last event is value 0, tested elsewhere */
+	while (libinput_next_event_type(li) != LIBINPUT_EVENT_NONE) {
+		ck_assert_int_eq(libinput_event_get_type(event),
+				 LIBINPUT_EVENT_POINTER_AXIS);
+		ptrev = libinput_event_get_pointer_event(event);
+
+		ck_assert_int_eq(libinput_event_pointer_get_axis(ptrev),
+				 LIBINPUT_POINTER_AXIS_SCROLL_VERTICAL);
+		ck_assert(libinput_event_pointer_get_axis_value(ptrev) > 0.0);
+
+		/* this is to verify we test the right thing, if the value
+		   is greater than scroll.threshold we triggered the wrong
+		   condition */
+		ck_assert(libinput_event_pointer_get_axis_value(ptrev) < 5.0);
+
+		libinput_event_destroy(event);
+		event = libinput_get_event(li);
+	}
+
+	litest_assert_empty_queue(li);
+	libinput_event_destroy(event);
+}
+END_TEST
+
 START_TEST(touchpad_scroll_natural_defaults)
 {
 	struct litest_device *dev = litest_current_device();
@@ -2073,6 +2117,7 @@ int main(int argc, char **argv) {
 	litest_add("touchpad:topsoftbuttons", clickpad_topsoftbuttons_move_out_ignore, LITEST_TOPBUTTONPAD, LITEST_ANY);
 
 	litest_add("touchpad:scroll", touchpad_2fg_scroll, LITEST_TOUCHPAD, LITEST_SINGLE_TOUCH);
+	litest_add("touchpad:scroll", touchpad_2fg_scroll_slow_distance, LITEST_TOUCHPAD, LITEST_SINGLE_TOUCH);
 	litest_add("touchpad:scroll", touchpad_scroll_natural_defaults, LITEST_TOUCHPAD, LITEST_ANY);
 	litest_add("touchpad:scroll", touchpad_scroll_natural_enable_config, LITEST_TOUCHPAD, LITEST_ANY);
 	litest_add("touchpad:scroll", touchpad_scroll_natural, LITEST_TOUCHPAD, LITEST_SINGLE_TOUCH);