touchpad: always reset the motion history on finger changes

We've already been doing this for semi-mt devices and for non-clickpads but
let's do it for clickpads as well. On Synaptics touchpads (PS/2 and RMI4)
we see slot jumps where two slots are active, slot X ends but slot Y continues
with the other slot's positional data. This causes a cursor jump on finger
lift after a two-finger scrolling motion. Simply resetting the motion history fixes it.

The only multi-finger interaction where a user could expect perfect fluid
motion is when using a second finger to touch cone of the software button
areas. Let's see if we have complaints first before we implement something
more complex.

https://bugs.freedesktop.org/show_bug.cgi?id=91695

Signed-off-by:Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
(cherry picked from commit aa87d2b25b88b86ddd1471f65e1afceff7fca9c4)

2 files changed, 56 insertions, 5 deletions

diff --git a/src/evdev-mt-touchpad.c b/src/evdev-mt-touchpad.c
index f15cfef6..eedbd3b8 100644
--- a/src/evdev-mt-touchpad.c
+++ b/src/evdev-mt-touchpad.c
@@ -955,11 +955,11 @@ tp_need_motion_history_reset(struct tp_dispatch *tp)
 {
 	bool rc = false;
 
-	/* Semi-mt finger postions may "jump" when nfingers changes. And on
-	 * a non-clickpad the only reason to have more than one finger down
-	 * is scrolling/gesture, so a reset just makes things sane again */
-	if ((tp->semi_mt || !tp->buttons.is_clickpad) &&
-	    tp->nfingers_down != tp->old_nfingers_down)
+	/* Changing the numbers of fingers can cause a jump in the
+	 * coordinates, always reset the motion history for all touches when
+	 * that happens.
+	 */
+	if (tp->nfingers_down != tp->old_nfingers_down)
 		return true;
 
 	/* if we're transitioning between slots and fake touches in either
diff --git a/test/touchpad.c b/test/touchpad.c
index bfdcab29..896d9c2f 100644
--- a/test/touchpad.c
+++ b/test/touchpad.c
@@ -4072,6 +4072,56 @@ START_TEST(touchpad_tool_tripletap_touch_count)
 }
 END_TEST
 
+START_TEST(touchpad_slot_swap)
+{
+	struct litest_device *dev = litest_current_device();
+	struct libinput *li = dev->libinput;
+	struct libinput_event *event;
+	struct libinput_event_pointer *ptrev;
+	int first, second;
+
+	/* Synaptics touchpads sometimes end the wrong touchpoint on finger
+	 * up, causing the remaining slot to continue with the other slot's
+	 * coordinates.
+	 * https://bugs.freedesktop.org/show_bug.cgi?id=91352
+	 */
+	litest_drain_events(li);
+
+	for (first = 0; first <= 1; first++) {
+		second = 1 - first;
+
+		litest_touch_down(dev, 0, 50, 50);
+		libinput_dispatch(li);
+		litest_touch_down(dev, 1, 70, 70);
+		libinput_dispatch(li);
+
+		litest_touch_move_to(dev, first, 50, 50, 50, 30, 10, 1);
+		litest_drain_events(li);
+
+		/* release touch 0, continue other slot with 0's coords */
+		litest_push_event_frame(dev);
+		litest_touch_up(dev, first);
+		litest_touch_move(dev, second, 50, 30.1);
+		litest_pop_event_frame(dev);
+		libinput_dispatch(li);
+		litest_touch_move_to(dev, second, 50, 30, 50, 11, 10, 1);
+		libinput_dispatch(li);
+		event = libinput_get_event(li);
+		do {
+			ptrev = litest_is_motion_event(event);
+			ck_assert_double_eq(libinput_event_pointer_get_dx(ptrev), 0.0);
+			ck_assert_double_lt(libinput_event_pointer_get_dy(ptrev), 1.0);
+
+			libinput_event_destroy(event);
+			event = libinput_get_event(li);
+		} while (event);
+		litest_assert_empty_queue(li);
+
+		litest_touch_up(dev, second);
+	}
+}
+END_TEST
+
 START_TEST(touchpad_time_usec)
 {
 	struct litest_device *dev = litest_current_device();
@@ -4265,6 +4315,7 @@ litest_setup_tests(void)
 	litest_add("touchpad:thumb", touchpad_thumb_tap_hold_2ndfg_tap, LITEST_CLICKPAD, LITEST_SINGLE_TOUCH);
 
 	litest_add_for_device("touchpad:bugs", touchpad_tool_tripletap_touch_count, LITEST_SYNAPTICS_TOPBUTTONPAD);
+	litest_add_for_device("touchpad:bugs", touchpad_slot_swap, LITEST_SYNAPTICS_TOPBUTTONPAD);
 
 	litest_add("touchpad:time", touchpad_time_usec, LITEST_TOUCHPAD, LITEST_ANY);