diff -Naur xfwm4-4.13.2/src/display.c xfwm4-4.13.2.slack-icon-bux-fixed/src/display.c
--- xfwm4-4.13.2/src/display.c	2019-05-15 15:35:15.000000000 -0400
+++ xfwm4-4.13.2.slack-icon-bux-fixed/src/display.c	2019-05-30 16:58:24.654061503 -0400
@@ -160,6 +160,7 @@
         "_NET_WM_WINDOW_TYPE_UTILITY",
         "_NET_WM_WINDOW_TYPE_NOTIFICATION",
         "_NET_WORKAREA",
+	"WM_CLASS",
         "MANAGER",
         "PIXMAP",
         "SM_CLIENT_ID",
diff -Naur xfwm4-4.13.2/src/display.h xfwm4-4.13.2.slack-icon-bux-fixed/src/display.h
--- xfwm4-4.13.2/src/display.h	2019-05-12 12:45:20.000000000 -0400
+++ xfwm4-4.13.2.slack-icon-bux-fixed/src/display.h	2019-05-30 16:52:52.369257999 -0400
@@ -252,6 +252,7 @@
     NET_WM_WINDOW_TYPE_UTILITY,
     NET_WM_WINDOW_TYPE_NOTIFICATION,
     NET_WORKAREA,
+    WM_CLASS,
     MANAGER,
     PIXMAP,
     SM_CLIENT_ID,
diff -Naur xfwm4-4.13.2/src/hints.c xfwm4-4.13.2.slack-icon-bux-fixed/src/hints.c
--- xfwm4-4.13.2/src/hints.c	2019-05-15 16:44:58.000000000 -0400
+++ xfwm4-4.13.2.slack-icon-bux-fixed/src/hints.c	2019-05-30 18:25:31.705949898 -0400
@@ -456,6 +456,7 @@
     atoms[i++] = display_info->atoms[NET_WM_WINDOW_TYPE_TOOLBAR];
     atoms[i++] = display_info->atoms[NET_WM_WINDOW_TYPE_UTILITY];
     atoms[i++] = display_info->atoms[NET_WORKAREA];
+    atoms[i++] = display_info->atoms[WM_CLASS];
     /* GTK specific hints */
     atoms[i++] = display_info->atoms[GTK_FRAME_EXTENTS];
     atoms[i++] = display_info->atoms[GTK_HIDE_TITLEBAR_WHEN_MAXIMIZED];
@@ -852,6 +853,23 @@
 }
 
 gboolean
+getWindowClass (DisplayInfo *display_info, Window w, gchar **name, gchar **class)
+{
+    XClassHint hint;
+    int status;
+    status = XGetClassHint (display_info->dpy, w, &hint);
+    if (status != 0) {
+	*class = g_strdup (hint.res_class);
+	*name = g_strdup (hint.res_name);
+	// Now free the x11 resource
+	XFree (hint.res_class);
+	XFree (hint.res_name);
+	return TRUE;
+    }
+    return FALSE;
+}
+
+gboolean
 getUTF8StringList (DisplayInfo *display_info, Window w, int atom_id, gchar ***str_p, guint *n_items)
 {
     char *xstr, *ptr;
diff -Naur xfwm4-4.13.2/src/hints.h xfwm4-4.13.2.slack-icon-bux-fixed/src/hints.h
--- xfwm4-4.13.2/src/hints.h	2019-05-12 12:45:20.000000000 -0400
+++ xfwm4-4.13.2.slack-icon-bux-fixed/src/hints.h	2019-05-30 17:33:28.019601707 -0400
@@ -298,4 +298,6 @@
                                                                  char **);
 #endif
 
+gboolean getWindowClass (DisplayInfo *, Window, gchar **, gchar **);
+
 #endif /* INC_HINTS_H */
diff -Naur xfwm4-4.13.2/src/icons.c xfwm4-4.13.2.slack-icon-bux-fixed/src/icons.c
--- xfwm4-4.13.2/src/icons.c	2019-05-12 12:45:20.000000000 -0400
+++ xfwm4-4.13.2.slack-icon-bux-fixed/src/icons.c	2019-05-30 18:44:15.217305112 -0400
@@ -535,7 +535,22 @@
             return icon;
         }
     }
-
+    else
+    {
+	gchar *name;
+	gchar *class;
+	if (getWindowClass (screen_info->display_info, window, &name, &class))
+	{
+	    GtkIconTheme *theme = gtk_icon_theme_get_default ();
+	    GdkPixbuf *icon = gtk_icon_theme_load_icon (theme, name, width, 0, NULL);
+	    g_free (name);
+	    g_free (class);
+	    if (icon) 
+	    {
+		return icon;
+	    }
+	}
+    }
     return default_icon_at_size (screen_info->gscr, width, height);
 }