Get a list of top-level windows on Wayland via ext_foreign_toplevel_list_v1. (I think it works, but... KWin doesn't implement the extension yet.)

ext-foreign-toplevel-list-v1.xml

Grab it at:

https://gitlab.freedesktop.org/wayland/wayland-protocols/-/tree/main/staging/ext-foreign-toplevel-list

Makefile

all: toplevels

toplevels: toplevels.c ext-foreign-toplevel-list-protocol.c ext-foreign-toplevel-list-protocol.h
        $(CC) -o toplevels ext-foreign-toplevel-list-protocol.c  toplevels.c -lwayland-client

ext-foreign-toplevel-list-protocol.h: ext-foreign-toplevel-list-v1.xml
        wayland-scanner client-header ext-foreign-toplevel-list-v1.xml ext-foreign-toplevel-list-protocol.h

ext-foreign-toplevel-list-protocol.c: ext-foreign-toplevel-list-v1.xml
        wayland-scanner private-code ext-foreign-toplevel-list-v1.xml ext-foreign-toplevel-list-protocol.c

clean:
        rm ext-foreign-toplevel-list-protocol.h ext-foreign-toplevel-list-protocol.c toplevels

toplevels.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wayland-client.h>
#include "ext-foreign-toplevel-list-protocol.h"

static struct wl_display *display = NULL;
static struct wl_registry *registry = NULL;
static struct ext_foreign_toplevel_list_v1 *toplevel_manager = NULL;

/*
void handle_geometry(void *data,
    struct ext_foreign_toplevel_handle_v1 *handle,
    int32_t x, int32_t y, int32_t width, int32_t height)
{
    printf("Geometry: x=%d y=%d w=%d h=%d\n", x, y, width, height);
}
*/

void handle_identifier(void *data,
    struct ext_foreign_toplevel_handle_v1 *handle,
    const char *identifier)
{
    printf("Identifier: %s\n", identifier);
}

void handle_app_id(void *data,
    struct ext_foreign_toplevel_handle_v1 *handle,
    const char *app_id)
{
    printf("App ID: %s\n", app_id);
}

void handle_title(void *data,
    struct ext_foreign_toplevel_handle_v1 *handle,
    const char *title)
{
    printf("Title: %s\n", title);
}

static const struct ext_foreign_toplevel_handle_v1_listener handle_listener = {
    .title = handle_title,
    .app_id = handle_app_id,
    .identifier = handle_identifier,
    //.geometry = handle_geometry,
};

static void handle_toplevel_handle(void *data,
    struct ext_foreign_toplevel_list_v1 *manager,
    struct ext_foreign_toplevel_handle_v1 *handle) {
    printf("Toplevel created\n");

    // You can add listeners to this handle to monitor state changes, titles, geometry, etc.
    ext_foreign_toplevel_handle_v1_add_listener(handle, &handle_listener, NULL);
}

static const struct ext_foreign_toplevel_list_v1_listener manager_listener = {
    .toplevel = handle_toplevel_handle,
    .finished = NULL
};

static void registry_handler(void *data, struct wl_registry *registry,
    uint32_t id, const char *interface, uint32_t version) {
    printf("Interface: %s (Version %d)\n", interface, version);
    if (strcmp(interface, "ext_foreign_toplevel_list_v1") == 0) {
        toplevel_manager = wl_registry_bind(registry, id,
            &ext_foreign_toplevel_list_v1_interface, 1);
        ext_foreign_toplevel_list_v1_add_listener(toplevel_manager, &manager_listener, NULL);
    }
}

static void registry_remove_handler(void *data, struct wl_registry *registry, uint32_t id) {}

static const struct wl_registry_listener registry_listener = {
    .global = registry_handler,
    .global_remove = registry_remove_handler
};

int main() {
    display = wl_display_connect(NULL);
    if (!display) {
        fprintf(stderr, "Failed to connect to Wayland display\n");
        return 1;
    }

    registry = wl_display_get_registry(display);
    wl_registry_add_listener(registry, &registry_listener, NULL);
    wl_display_roundtrip(display);

    if (!toplevel_manager) {
        fprintf(stderr, "Wayland-compatible compositor does not (yet) implement ext_foreign_toplevel_list_v1.\n");
        wl_display_disconnect(display);
        return 2;
    }

    // Main event loop
    while (wl_display_dispatch(display) != -1) {
        // Wait for events
    }

    wl_display_disconnect(display);
    return 0;
}