When building with -Wno-deprecated-declarations the warnings listed below remain.
=> so it seems the use of an incompatible type is the culprit here. tray.c:634:19: error: assignment to ‘GtkWidget *’ {aka ‘struct _GtkWidget *’} from incompatible pointer type ‘GObject *’ {aka ‘struct _GObject *’} [-Wincompatible-pointer-types] 634 | tr->invisible = g_object_ref_sink(G_OBJECT(invisible)); | ^ See https://github.com/lxde/lxpanel/commit/0853b0fc981285ebd2ac52f8dfc2a09b1090748c for the corresponding upstream fix.