spice-widget: fix hotspot position on X11/HiDPI

GDK scales hotspot coordinates using screen scale factor on X11. We need
to undo this to get the correct hotspot behavior, otherwise mouse
selection becomes very inaccurate and difficult in the guest.

See [1].

[1] https://gitlab.gnome.org/GNOME/gtk/-/blob/3.24.21/gdk/x11/gdkcursor-x11.c#L556-557

Signed-off-by: Haochen Tong <i@hexchain.org>
Acked-by: Frediano Ziglio <fziglio@redhat.com>

src/spice-widget.c

@@ -2964,6 +2964,7 @@ static void cursor_set(SpiceCursorChannel *channel,
     SpiceDisplayPrivate *d = display->priv;
     GdkCursor *cursor = NULL;
     SpiceCursorShape *cursor_shape;
+    gint hotspot_x, hotspot_y;
 
     g_object_get(G_OBJECT(channel), "cursor", &cursor_shape, NULL);
     if (G_UNLIKELY(cursor_shape == NULL || cursor_shape->data == NULL)) {
@@ -2985,12 +2986,22 @@ static void cursor_set(SpiceCursorChannel *channel,
                                                cursor_shape_destroy, cursor_shape);
     d->cursor_surface = gdk_cairo_surface_create_from_pixbuf(d->mouse_pixbuf, 0,
                                                              gtk_widget_get_window(GTK_WIDGET(display)));
-    d->mouse_hotspot.x = cursor_shape->hot_spot_x;
-    d->mouse_hotspot.y = cursor_shape->hot_spot_y;
+    hotspot_x = d->mouse_hotspot.x = cursor_shape->hot_spot_x;
+    hotspot_y = d->mouse_hotspot.y = cursor_shape->hot_spot_y;
+
+#ifdef GDK_WINDOWING_X11
+    /* undo hotspot scaling in gdkcursor */
+    if (GDK_IS_X11_DISPLAY(gtk_widget_get_display(GTK_WIDGET(display)))) {
+        gint scale_factor = gdk_window_get_scale_factor(gtk_widget_get_window(GTK_WIDGET(display)));
+        hotspot_x /= scale_factor;
+        hotspot_y /= scale_factor;
+    }
+#endif
+
     cursor = gdk_cursor_new_from_surface(gtk_widget_get_display(GTK_WIDGET(display)),
                                          d->cursor_surface,
-                                         d->mouse_hotspot.x,
-                                         d->mouse_hotspot.y);
+                                         hotspot_x,
+                                         hotspot_y);
 
 #if HAVE_EGL
     if (egl_enabled(d))