fw/services/touch: release an active touch when the last subscriber leaves

The last subscriber going away powers the touch sensor down mid-gesture,
so a finger that was down at teardown never produces a Liftoff. The
Touchdown's backlight hold (light_touch_down -> light_button_pressed)
then never unwinds: the button refcount stays pinned, the light never
enters its timed state, and with the sensor off no later touch can clear
it.

Synthesise the Liftoff with the last coordinates when the subscriber
count reaches zero, mirroring the teardown the global disable path
already performs for the same reason.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Joseph Mearman <joseph@mearman.co.uk>

Author: Mearman

src/fw/services/touch/touch.c

@@ -63,19 +63,22 @@ static void prv_add_subscriber_cb(PebbleTask task) {
 static void prv_remove_subscriber_cb(PebbleTask task) {
   mutex_lock(s_touch_mutex);
   PBL_ASSERTN(s_subscriber_count > 0);
-  if (--s_subscriber_count == 0 && s_globally_enabled) {
+  const bool was_last_subscriber = (--s_subscriber_count == 0);
+  if (was_last_subscriber && s_globally_enabled) {
     touch_sensor_set_enabled(false);
   }
-  // An app whose shared touch subscription disappears (task exit, or both
-  // slots emptied) cannot have a live raw handler or nav dispatcher anymore;
-  // drop its raw-slot mark and the app-nav-active flag so a crashed app cannot
-  // leak backlight-follow behavior.
+  // An app whose shared touch subscription disappears (task exit, or both slots emptied) cannot have a live raw handler or nav dispatcher anymore; drop its raw-slot mark and the app-nav-active flag so a crashed app cannot leak backlight-follow behavior.
   if (task == PebbleTask_App) {
     s_raw_subscriber_tasks &= (uint8_t)~(1u << task);
     s_app_nav_active = false;
   }
   PBL_LOG_DBG("Touch: subscriber removed, count=%" PRIu8, s_subscriber_count);
   mutex_unlock(s_touch_mutex);
+
+  if (was_last_subscriber) {
+    // A finger down when the last subscriber goes away never gets a Liftoff otherwise (the sensor stops reporting), so the backlight hold counter stays pinned. Same teardown the global toggle performs.
+    touch_release_active();
+  }
 }
 
 void touch_init(void) {

tests/fw/services/test_touch.c

@@ -591,6 +591,26 @@ void test_touch__reset_mid_gesture_refuses_continuation(void) {
   touch_handle_injected_update(TouchInjectPhase_End, 50, 60);
 }
 
+void test_touch__last_unsubscribe_with_finger_down_emits_liftoff(void) {
+  // Finger down, then the last subscriber goes away: the sensor powers down mid-gesture, so a Liftoff must be synthesized with the last coordinates (not zeros) or the backlight hold taken by the Touchdown never unwinds — the same teardown the global toggle performs.
+  s_add_subscriber_cb(PebbleTask_App);
+  touch_handle_update(TouchState_FingerDown, 30, 40);
+  fake_event_reset_count();
+
+  s_remove_subscriber_cb(PebbleTask_App);
+  cl_assert_equal_i(fake_event_get_count(), 1);
+  prv_assert_touch_event(TouchEvent_Liftoff, 30, 40);
+}
+
+void test_touch__last_unsubscribe_without_finger_no_liftoff(void) {
+  // No finger down: the last subscriber leaving must not fabricate a Liftoff.
+  s_add_subscriber_cb(PebbleTask_App);
+  fake_event_reset_count();
+
+  s_remove_subscriber_cb(PebbleTask_App);
+  cl_assert_equal_i(fake_event_get_count(), 0);
+}
+
 void test_touch__event_abi_unchanged(void) {
   // non_navigational rides in the padding after type:8; x/y offsets and the
   // overall size must not move, keeping the SDK struct app-compatible.