[Gui] change cursor to avoid issues under WSLg

See #2586.
Avoids issues when running WSLg under Ubuntu 20.04, with lablgtk3, on
Wayland, due to missing cursors (such as ARROW).

src/plugins/gui/design.ml

@@ -862,7 +862,14 @@ class main_window () : main_window_extension_points =
   let () = main_window#set_default_size ~width ~height in
   let () = main_window#set_geometry_hints ~min_size:(1,1) main_window#coerce in
   let watch_cursor = Gdk.Cursor.create `WATCH in
-  let arrow_cursor = Gdk.Cursor.create `ARROW in
+  (* NOTE: the ARROW cursor is not available under some specific configurations,
+     e.g. WSLg + lablgtk3 + Wayland; so we avoid using it here.
+     Testing indicates only the following cursors are available:
+     BOTTOM_LEFT_CORNER, BOTTOM_RIGHT_CORNER, BOTTOM_SIDE, CROSSHAIR, HAND1,
+     LEFT_PTR, LEFT_SIDE, RIGHT_SIDE, TOP_LEFT_CORNER, TOP_RIGHT_CORNER,
+     TOP_SIDE, WATCH, XTERM
+  *)
+  let arrow_cursor = Gdk.Cursor.create `LEFT_PTR in
 
   (* On top one finds the menubar *)
   let toplevel_vbox = GPack.box `VERTICAL ~packing:main_window#add () in