FLTK implementation of the "GTK Shell" Wayland protocol - cont'd

The pointer_enter() function now checks that its non-FLTK wl_surface  argument
is the wl_surface of the titlebar of a GTK-decorated window.

libdecor/build/fl_libdecor-plugins.c

@@ -236,13 +236,7 @@ static unsigned char *gtk_titlebar_buffer(struct libdecor_frame *frame,
                                           int *width, int *height, int *stride)
 {
   struct libdecor_frame_gtk *lfg = (struct libdecor_frame_gtk *)frame;
-#if USE_SYSTEM_LIBDECOR || !HAVE_GTK
+  struct buffer *buffer = lfg->headerbar.buffer;
-  struct border_component_gtk *bc;
-#else
-  struct border_component *bc;
-#endif
-  bc = &lfg->headerbar;
-  struct buffer *buffer = bc->buffer;
   *width = buffer->buffer_width;
   *height = buffer->buffer_height;
   *stride = cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, buffer->buffer_width);
@@ -313,6 +307,15 @@ static const char *get_libdecor_plugin_description(struct libdecor_frame *frame)
 }
 
 
+static const char *plugin_name(struct libdecor_frame *frame) {
+  static const char *my_plugin = NULL;
+  if (!my_plugin) {
+    my_plugin = get_libdecor_plugin_description(frame);
+    if (!my_plugin) my_plugin = "unknown";
+  }
+  return my_plugin;
+}
+
 /*
  FLTK-added utility function to give access to the pixel array representing
  the titlebar of a window decorated by the cairo plugin of libdecor.
@@ -324,13 +327,22 @@ static const char *get_libdecor_plugin_description(struct libdecor_frame *frame)
 unsigned char *fl_libdecor_titlebar_buffer(struct libdecor_frame *frame,
                                                  int *width, int *height, int *stride)
 {
-  static const char *my_plugin = NULL;
+  const char *name = plugin_name(frame);
-  if (!my_plugin) my_plugin = get_libdecor_plugin_description(frame);
+  if (!strcmp(name, "GTK3 plugin")) {
-  if (my_plugin && !strcmp(my_plugin, "GTK3 plugin")) {
     return gtk_titlebar_buffer(frame, width, height, stride);
   }
-  else if (my_plugin && !strcmp(my_plugin, "libdecor plugin using Cairo")) {
+  else if (!strcmp(name, "libdecor plugin using Cairo")) {
     return cairo_titlebar_buffer(frame, width, height, stride);
   }
   return NULL;
 }
+
+
+/* returns the wl_surface of the window's headerbar for GTK */
+struct wl_surface *fl_headerbar_surface(struct libdecor_frame *frame) {
+  if (!strcmp(plugin_name(frame), "GTK3 plugin")) {
+    struct libdecor_frame_gtk *lfg = (struct libdecor_frame_gtk *)frame;
+    return lfg->headerbar.wl_surface;
+  }
+  return NULL;
+}

src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx

@@ -47,6 +47,7 @@
 #include <string.h> // for strerror()
 extern "C" {
   bool libdecor_get_cursor_settings(char **theme, int *size);
+  struct wl_surface *fl_headerbar_surface(struct libdecor_frame *frame);
 }
 
 
@@ -201,7 +202,18 @@ static Fl_Window *event_coords_from_surface(struct wl_surface *surface,
 static void pointer_enter(void *data, struct wl_pointer *wl_pointer, uint32_t serial,
         struct wl_surface *surface, wl_fixed_t surface_x, wl_fixed_t surface_y) {
   Fl_Window *win = event_coords_from_surface(surface, surface_x, surface_y);
-  if (!win && gtk_shell) gtk_shell_surface = surface;
+  if (!win && gtk_shell) { // check that surface is the headerbar of a GTK-decorated window
+    Fl_X *x = Fl_X::first;
+    while (x) {
+      struct wld_window *xid = (struct wld_window*)x->xid;
+      if (xid->kind == Fl_Wayland_Window_Driver::DECORATED &&
+          surface == fl_headerbar_surface(xid->frame)) {
+        gtk_shell_surface = surface;
+        return;
+      }
+      x = x->next;
+    }
+  }
   if (!win) return;
   // use custom cursor if present
   struct wl_cursor *cursor =