fix: retain the lower player slot when joining Joy-Cons

ADAPTER_PARITY_PLAN.md

@@ -678,6 +678,13 @@ all chord buttons must release before retry. Confirmation is bounded to the
 participating halves. Disconnect/reuse or a changed saved default clears the
 override, without writing a per-pair preference.
 
+Explicit gesture joins retain the lower participating player slot rather than
+always retaining the left half's slot. A regression reproduced the old player-2
+result with the left half ready first but occupying the higher transport/player
+index. Both ready orders and an earlier unrelated player are covered. Hardware
+with right on player 1 and left on player 2 now joins on player 1; all profile
+banks remain unchanged.
+
 All 349 tests and all five builds passed. Real hardware exercised both live
 mode transitions, Individual boot persistence, physical join/split pulses,
 unchanged configuration generations during gestures, and automatic default

README.md

@@ -237,6 +237,9 @@ uv run switch-pico-config joycon-mode paired --json
   pulse, then **release all four buttons**.
 
 Two solo halves join; an existing pair splits. Other controllers are untouched.
+An explicit join keeps the lower of the two participating player slots, not
+whichever slot belongs to the left half. Both LEDs follow that retained slot;
+the other slot is neutralized. It never takes a slot from an unrelated player.
 The shortcut does not change the saved default: its connection-only override
 ends when either participating half disconnects, or when the adapter default
 changes. Existing solo/pair banks and active selections are reused.

src/firmware/input/bluepad32_input_backend.cpp

@@ -2665,8 +2665,14 @@ void process_joycon_gestures(uint32_t now_ms) {
         if (!mature) continue;
         // Latch success AND failure before any enrollment I/O. A failed seed
         // must not retry at the timer cadence or undo either participant.
+        // Player-slot ownership is independent of physical handedness.
+        // Keep the lower occupied slot, even when the left half owns the higher one.
+        const bool keep_left_slot = owner_slot < right_slot;
         const bool changed = joining
-            ? merge_joycon_slots(owner_slot, right_slot, right, true)
+            ? merge_joycon_slots(
+                  keep_left_slot ? owner_slot : right_slot,
+                  keep_left_slot ? right_slot : owner_slot,
+                  keep_left_slot ? right : gesture.device, true)
             : split_joycon_slot(static_cast<uint8_t>(owner_slot), true);
         if (changed) recompute_connection_status();
     }

tests/bluepad32_backend_lifecycle_test.cpp

@@ -1679,6 +1679,41 @@ void test_switch2_gesture_timing() {
             "connection gestures must never write the saved adapter preference");
 }
 
+void test_switch2_gesture_slot_order(bool left_first, uint8_t lower_slot) {
+    runtime_configuration.joycon_mode = JoyConMode::kIndividual;
+    start_backend();
+    auto ordinary = device(0, true, UNI_BT_CONN_PROTOCOL_BR_EDR);
+    auto left = switch2_device(lower_slot + 1, UNI_SW2_JOYCON_L_PID);
+    auto right = switch2_device(lower_slot, UNI_SW2_JOYCON_R_PID);
+    if (lower_slot != 0) ready_switch2(ordinary);
+    // Transport indices can put the left half in the higher player slot,
+    // even when its ready callback arrives first.
+    ready_switch2(left_first ? left : right);
+    ready_switch2(left_first ? right : left);
+    const auto lower_before = slot_snapshot(lower_slot);
+    const auto higher_before = slot_snapshot(lower_slot + 1);
+    const auto unrelated_before = slot_snapshot(0);
+    hold_gesture(left, right, 2000);
+    require(live_pair_slot(left, right) == lower_slot &&
+                !slot_snapshot(lower_slot + 1).active,
+            "gesture join must retain the lower participating player slot regardless of side or ready order");
+    const auto joined = slot_snapshot(lower_slot);
+    require_pair_owner(joined.identity, left, right);
+    require(joined.connection_generation != lower_before.connection_generation &&
+                slot_snapshot(lower_slot + 1).connection_generation != higher_before.connection_generation &&
+                slot_snapshot(lower_slot + 1).state.left_stick_x == 0 &&
+                slot_snapshot(lower_slot + 1).state.right_stick_x == 0 &&
+                left.player_leds == (1u << lower_slot) &&
+                right.player_leds == (1u << lower_slot),
+            "join must invalidate old player epochs, neutralize the retired slot and light both halves for the retained player");
+    if (lower_slot != 0) {
+        require(slot_snapshot(0).active &&
+                    slot_snapshot(0).connection_generation == unrelated_before.connection_generation &&
+                    controller_identity_equal(slot_snapshot(0).identity, unrelated_before.identity),
+                "gesture join must not take an earlier slot occupied by an unrelated controller");
+    }
+}
+
 void test_switch2_gesture_stale() {
     runtime_configuration.joycon_mode = JoyConMode::kIndividual;
     start_backend();
@@ -5377,6 +5412,12 @@ int main(int argc, char** argv) {
         test_switch2_hd_pro();
     } else if (scenario == "switch2-gesture-timing") {
         test_switch2_gesture_timing();
+    } else if (scenario == "switch2-gesture-slot-left-first") {
+        test_switch2_gesture_slot_order(true, 0);
+    } else if (scenario == "switch2-gesture-slot-right-first") {
+        test_switch2_gesture_slot_order(false, 0);
+    } else if (scenario == "switch2-gesture-slot-occupied") {
+        test_switch2_gesture_slot_order(true, 1);
     } else if (scenario == "switch2-gesture-stale") {
         test_switch2_gesture_stale();
     } else if (scenario == "switch2-gesture-clock-wrap") {

tests/test_bluepad32_backend_lifecycle_native.py

@@ -108,6 +108,9 @@ def test_bluepad32_backend_lifecycle_native(tmp_path: Path) -> None:
             "switch2-mode-seed-failure",
             "switch2-mode-identity-failure",
             "switch2-gesture-timing",
+            "switch2-gesture-slot-left-first",
+            "switch2-gesture-slot-right-first",
+            "switch2-gesture-slot-occupied",
             "switch2-gesture-stale",
             "switch2-gesture-clock-wrap",
             "switch2-gesture-masking-epochs",