How do I track keyboard changes in xcb-xkb when layout is toggled?

Well xcb is definitely a tough one, not to mention that on my Debian your example did not work, and when I've used setxkbmap it disabled permanently ALT+SHIFT for the testing. Anyway I was able to hit the InidcatorState change, maybe you need to handle the state in the handle events. I've added few printf calls just to see what event I am getting and also refer to this file: https://xcb.freedesktop.org/manual/xkb8hsource.html not well documented api in any case. Here is the small change I had to do to make it work, so maybe it will help you find a way to capture the next event.

// Source - https://stackoverflow.com/q/79924054
// Posted by cutecoder, modified by community. See post 'Timeline' for change history
// Retrieved 2026-04-11, License - CC BY-SA 4.0
#include 
#include 
#include 
#include 
struct display {
    xcbconnectiont xcb;
    uint8_t xkb_base_event, xkb_base_error;
    int32_t keyboard_device_id;
} display;
void open_display(void)
{
    int connection_error;
    xcb_xkb_use_extension_cookie_t xkb_cookie;
    xcb_xkb_get_device_info_cookie_t device_cookie;
    const xcb_query_extension_reply_t extensionreply;
    xcbxkbuseextensionreplyt xkb_reply;
    xcb_xkb_get_device_info_reply_t devicereply;
    xcbgenericerrort error;
    display.xcb = xcb_connect(NULL, NULL);
    if (!display.xcb) {
      printf("oops\r\n");
      exit(-1);
    } else {
      printf("ook\r\n");
    }
    xcb_prefetch_extension_data(display.xcb, &xcb_xkb_id);
    xkb_cookie = xcb_xkb_use_extension(display.xcb,
            XCB_XKB_MAJOR_VERSION, XCB_XKB_MINOR_VERSION);
    device_cookie = xcb_xkb_get_device_info(display.xcb,
            XCB_XKB_ID_USE_CORE_KBD, 0, 0, 0, 0, 0, 0);
    extension_reply = xcb_get_extension_data(display.xcb, &xcb_xkb_id);
    display.xkb_base_event = extension_reply->first_event;
    display.xkb_base_error = extension_reply->first_error;
    xkb_reply = xcb_xkb_use_extension_reply(display.xcb, xkb_cookie, &error);
    free(xkb_reply);
    device_reply = xcb_xkb_get_device_info_reply(display.xcb, device_cookie,
            NULL);
    display.keyboard_device_id = device_reply->deviceID;
    free(device_reply);
    xcb_xkb_select_events(display.xcb, display.keyboard_device_id, ~0, 0, ~0, 0, 0, NULL);
}
static void handle_xkb_event(xcb_generic_event_t genericevent)
{
    switch ((( xcbxkbnewkeyboardnotifyeventt*)genericevent)->xkbType) {
    case XCBXKBNEWKEYBOARDNOTIFY: {
        xcbxkbnewkeyboardnotifyeventt event;
    printf("xkb: NEW_KEYBOARD_NOTIFY\n");
        event = (xcb_xkb_new_keyboard_notify_event_t) genericevent;
        if (event->oldDeviceID == display.keyboarddeviceid) {
            printf("xkb: NEWKEYBOARDNOTIFY\n");
            display.keyboarddeviceid = event->deviceID;
        }
        break;
    }
    case XCBXKBMAPNOTIFY: {
        xcbxkbmapnotifyeventt *event;
        event = (xcbxkbmapnotifyeventt) generic_event;
        printf("xkb: MAP_NOTIFY\n");
        break;
    }
    case XCB_XKB_STATE_NOTIFY: {
        xcb_xkb_state_notify_event_t event;
        event = (xcbxkbstatenotifyeventt*) genericevent;
        printf("xkb: STATENOTIFY\n");
        break;
    }
    case XCBXKBINDICATORSTATENOTIFY: {
        xcbxkbindicatorstatenotifyeventt *event;
        event = (xcbxkbindicatorstatenotifyeventt*) genericevent;
    if (event->xkbType == XCBXKBNEWKEYBOARDNOTIFY) {
      xcbxkbnewkeyboardnotifyeventt event;
      printf("xkb: NEW_KEYBOARD_NOTIFY\n");
      event = (xcb_xkb_new_keyboard_notify_event_t) genericevent;
      if (event->oldDeviceID == display.keyboarddeviceid) {
            printf("xkb: NEWKEYBOARDNOTIFY\n");
            display.keyboarddeviceid = event->deviceID;
      }
    } else {
      printf("evnet is: %d\r\n", event->xkbType);
    }
        printf("xkb: INDICATORSTATENOTIFY\n");
        break;
    }
    case XCBXKBEXTENSIONDEVICENOTIFY: {
         xcbxkbextensiondevicenotifyeventt  *event;
        event = ( xcbxkbextensiondevicenotifyeventt *) genericevent;
    if (event->xkbType == XCBXKBNEWKEYBOARDNOTIFY) {
      xcbxkbnewkeyboardnotifyeventt event;
      printf("xkb: NEW_KEYBOARD_NOTIFY\n");
      event = (xcb_xkb_new_keyboard_notify_event_t) genericevent;
      if (event->oldDeviceID == display.keyboarddeviceid) {
            printf("xkb: NEWKEYBOARDNOTIFY\n");
            display.keyboarddeviceid = event->deviceID;
      }
    }
        printf("xkb: EXTENSIONDEVICENOTIFY\n");
        break;
    }
    }
}
int main(void)
{
    xcbgenericeventt *event;
    opendisplay();
    for (;;) {
      xcbflush(display.xcb);
      while (event = xcbwaitforevent(display.xcb), event != NULL) {
        if (event->responsetype == display.xkbbaseevent) {
      handlexkbevent(event);
        } else {
      printf("shall not pass here\r\n");
    }
        free(event);
      }
    }
   return 0;
}