fix(usb): add suspend-timeout fallback for boards without VBUS detect

app/Kconfig

@@ -154,6 +154,18 @@ config USB_NUMOF_EP_WRITE_RETRIES
 config USB_HID_POLL_INTERVAL_MS
     default 1
 
+config ZMK_USB_SUSPEND_DISCONNECT_TIMEOUT_MS
+    int "USB suspend disconnect timeout in milliseconds"
+    default 0
+    help
+      When non-zero, treat a USB suspend lasting longer than this many
+      milliseconds as a disconnect.  This is useful for boards without
+      VBUS detection where the USB controller reports USB_DC_SUSPEND
+      when the cable is physically unplugged, preventing the endpoint
+      from switching to BLE.
+
+      Set to 0 (default) to disable this workaround.
+
 endif # ZMK_USB
 
 menuconfig ZMK_BLE

app/boards/lowprokb/corneish_zen/Kconfig.defconfig

@@ -33,6 +33,13 @@ config USB_NRFX
 config USB_DEVICE_STACK
     default y
 
+# The Zen board has no VBUS detection.  When the USB cable is
+# unplugged, the nRF52840 reports USB_DC_SUSPEND, which ZMK treats as
+# "still connected."  A 2-second timeout causes the keyboard to fall
+# back to BLE after cable disconnect.
+config ZMK_USB_SUSPEND_DISCONNECT_TIMEOUT_MS
+    default 2000
+
 endif # USB
 
 config ZMK_DISPLAY

app/src/usb.c

@@ -22,6 +22,33 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 static enum usb_dc_status_code usb_status = USB_DC_UNKNOWN;
 static bool is_configured;
 
+#define SUSPEND_TIMEOUT_ACTIVE (CONFIG_ZMK_USB_SUSPEND_DISCONNECT_TIMEOUT_MS > 0)
+
+#if SUSPEND_TIMEOUT_ACTIVE
+
+static void usb_suspend_timeout_fn(struct k_timer *timer);
+static K_TIMER_DEFINE(usb_suspend_timer, usb_suspend_timeout_fn, NULL);
+
+static void usb_suspend_timeout_fn(struct k_timer *timer) {
+    // If we are still in SUSPEND after the timeout, the cable was
+    // likely unplugged (no VBUS detection).  Treat as disconnect so
+    // the endpoint falls back to BLE.
+    if (usb_status == USB_DC_SUSPEND) {
+        usb_status = USB_DC_DISCONNECTED;
+        is_configured = false;
+        k_work_submit(&usb_status_notifier_work);
+    }
+}
+
+static inline void suspend_timeout_start(void) {
+    k_timer_start(&usb_suspend_timer, K_MSEC(CONFIG_ZMK_USB_SUSPEND_DISCONNECT_TIMEOUT_MS),
+                  K_NO_WAIT);
+}
+
+static inline void suspend_timeout_cancel(void) { k_timer_stop(&usb_suspend_timer); }
+
+#endif
+
 static void raise_usb_status_changed_event(struct k_work *_work) {
     raise_zmk_usb_conn_state_changed(
         (struct zmk_usb_conn_state_changed){.conn_state = zmk_usb_get_conn_state()});
@@ -66,6 +93,15 @@ void usb_status_cb(enum usb_dc_status_code status, const uint8_t *params) {
         zmk_usb_hid_set_protocol(HID_PROTOCOL_REPORT);
     }
 #endif
+
+#if SUSPEND_TIMEOUT_ACTIVE
+    if (status == USB_DC_SUSPEND) {
+        suspend_timeout_start();
+    } else {
+        suspend_timeout_cancel();
+    }
+#endif
+
     usb_status = status;
     if (zmk_usb_get_conn_state() == ZMK_USB_CONN_HID) {
         is_configured |= usb_status == USB_DC_CONFIGURED;