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The following code demonstrates that the state reported is always 0, even when pressing down a modifier (it works on Xorg and Xwayland):

#include <stdlib.h>

#include <gtk/gtk.h>

static void
log_device_state_for_drop (GdkDrop *drop)
{
    GdkDevice *device;
    GdkSurface *surface;
    GdkModifierType state;

    device = gdk_drop_get_device (drop);
    surface = gdk_drop_get_surface (drop);

    gdk_device_get_state (device, surface, NULL, &state);

    state &= gtk_accelerator_get_default_mod_mask ();

    g_message ("%s(): device state: %u", __func__, state);
}

static void
on_drag_motion (GtkWidget *widget,
                GdkDrop   *drop,
                int        x,
                int        y,
                gpointer   user_data)
{
    g_message ("%s()", __func__);

    log_device_state_for_drop (drop);
}

static void
on_drag_data_received (GtkWidget        *widget,
                       GdkDrop          *drop,
                       GtkSelectionData *data,
                       gpointer          user_data)
{
    g_message ("%s()", __func__);

    log_device_state_for_drop (drop);
}

int
main (int    argc,
      char **argv)
{
    GtkWidget *window;
    GtkWidget *grid;
    g_autoptr (GdkContentFormats) formats;
    GtkWidget *source;
    GtkWidget *separator;
    GtkWidget *destination;

    gtk_init ();

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    grid = gtk_grid_new ();
    formats = gdk_content_formats_new (NULL, 0);
    formats = gtk_content_formats_add_text_targets (formats);
    source = gtk_label_new ("Source");
    separator = gtk_separator_new (GTK_ORIENTATION_HORIZONTAL);
    destination = gtk_label_new ("Destination");

    g_signal_connect (window, "close-request", G_CALLBACK (gtk_main_quit), NULL);
    gtk_container_add (GTK_CONTAINER (window), grid);
    gtk_widget_show (window);

    gtk_orientable_set_orientation (GTK_ORIENTABLE (grid), GTK_ORIENTATION_VERTICAL);
    gtk_container_add (GTK_CONTAINER (grid), source);
    gtk_container_add (GTK_CONTAINER (grid), separator);
    gtk_container_add (GTK_CONTAINER (grid), destination);

    g_object_set (source, "expand", TRUE, NULL);
    gtk_drag_source_set (source, GDK_BUTTON1_MASK, formats, GDK_ACTION_ALL);

    g_object_set (destination, "expand", TRUE, NULL);
    g_signal_connect (destination, "drag-motion", G_CALLBACK (on_drag_motion), NULL);
    g_signal_connect (destination, "drag-data-received", G_CALLBACK (on_drag_data_received), NULL);
    gtk_drag_dest_set (destination, GTK_DEST_DEFAULT_ALL, formats, GDK_ACTION_ALL);

    gtk_main ();

    return EXIT_SUCCESS;
}