Track X11 enter / leave events

I need to be notified when mouse enters or leaves a window (any window, that's why I'm grabbing on the root window). I have a solution using xcb_input_xi_grab_device() but it works only half way. When I run the app, I do get enter/leave events but the mouse pointer disappears and it consumes all mouse events and other windows stop (children of the root window) reacting to clicks.

Using XCB_INPUT_GRAB_OWNER_OWNER should allow passing-through the events but it doesn't work.

app.c:

#include <stdio.h>
#include <xcb/xcb.h>
#include <xcb/xinput.h>

xcb_input_device_id_t find_device(xcb_connection_t *conn)
{
    xcb_input_xi_query_device_reply_t *reply = xcb_input_xi_query_device_reply(
        conn, xcb_input_xi_query_device(conn, XCB_INPUT_DEVICE_ALL), NULL);
    xcb_input_xi_device_info_iterator_t iter =
        xcb_input_xi_query_device_infos_iterator(reply);

    xcb_input_device_id_t device_id;

    int found = 0;
    while (iter.rem) {
        xcb_input_xi_device_info_t *device = iter.data;
        switch (device->type) {
            case XCB_INPUT_DEVICE_TYPE_MASTER_POINTER:
            case XCB_INPUT_DEVICE_TYPE_SLAVE_POINTER:
            case XCB_INPUT_DEVICE_TYPE_FLOATING_SLAVE:
                device_id = device->deviceid;
                found = 1;
                break;
        }
        if (found) {
            break;
        }
        xcb_input_xi_device_info_next(&iter);
    }
    free(reply);

    return device_id;
}

int main()
{
    xcb_connection_t *conn = xcb_connect(NULL, NULL);
    xcb_screen_t *screen = xcb_setup_roots_iterator(xcb_get_setup(conn)).data;

    uint32_t mask =
        XCB_INPUT_XI_EVENT_MASK_ENTER | XCB_INPUT_XI_EVENT_MASK_LEAVE;
    xcb_generic_error_t *error;
    xcb_input_xi_grab_device_reply_t *reply = xcb_input_xi_grab_device_reply(
        conn,
        xcb_input_xi_grab_device(conn, screen->root, XCB_CURRENT_TIME,
                                 XCB_CURSOR_NONE, find_device(conn),
                                 XCB_INPUT_GRAB_MODE_22_ASYNC,
                                 XCB_INPUT_GRAB_MODE_22_ASYNC,
                                 XCB_INPUT_GRAB_OWNER_OWNER, 1, &mask),
        &error);
    free(reply);
    if (error) {
        printf("failed to grab device\n");
        free(error);
        return -1;
    }

    xcb_flush(conn);

    xcb_generic_event_t *event;
    while ((event = xcb_wait_for_event(conn))) {
        xcb_ge_event_t *generic_event = event;
        switch (generic_event->event_type) {
            case XCB_INPUT_ENTER:
                printf("enter\n");
                break;
            case XCB_INPUT_LEAVE:
                printf("leave\n");
                break;
        }
        free(event);
    }

    return -1;  // never reached
}

cc -O2 app.c -o app `pkg-config --cflags --libs xcb xcb-xinput`

I also tried to achieve it with xcb_grab_button(), xcb_input_xi_select_events(), xcb_input_xi_passive_grab_device() or xcb_grab_pointer() but then the problem is the same. Either the mouse events are not passed-through or I don't receive enter/leave events at all.

> I need to be notified when mouse enters or leaves a window

I would implement that by selecting enter and leave events on all windows (event masks EnterWindow and LeaveWindow).

The "all windows" part would of course need to be dynamic: When new windows are created, they also need to be monitored. Thus, you also need SubstructureNotify so that you get CreateNotify events when new windows are created.