Fix recalculation of `icon-title-format' after a frame is iconified

Previously it would only happen upon the next mode line
redisplay, meaning that just pressing C-z would not update the
implicit title, since C-z doesn't cause a redisplay.

* src/dispextern.h: Update prototypes.
* src/frame.h (SET_FRAME_ICONIFIED): De-slugify.  Call
`gui_consider_frame_title', since `icon-title-format' might be
different from the current frame title.
* src/xdisp.c (gui_consider_frame_title): Export (also in
dispextern.h).  (bug#55850)

src/dispextern.h

@@ -3470,6 +3470,7 @@ extern void expose_frame (struct frame *, int, int, int, int);
 extern bool gui_intersect_rectangles (const Emacs_Rectangle *,
                                       const Emacs_Rectangle *,
                                       Emacs_Rectangle *);
+extern void gui_consider_frame_title (Lisp_Object);
 #endif	/* HAVE_WINDOW_SYSTEM */
 
 extern void note_mouse_highlight (struct frame *, int, int);

src/frame.h

@@ -1293,8 +1293,28 @@ SET_FRAME_VISIBLE (struct frame *f, int v)
 }
 
 /* Set iconified status of frame F.  */
-#define SET_FRAME_ICONIFIED(f, i)				\
-  (f)->iconified = (eassert (0 <= (i) && (i) <= 1), (i))
+INLINE void
+SET_FRAME_ICONIFIED (struct frame *f, int i)
+{
+#ifdef HAVE_WINDOW_SYSTEM
+  Lisp_Object frame;
+#endif
+
+  eassert (0 <= (i) && (i) <= 1);
+
+  f->iconified = i;
+
+#ifdef HAVE_WINDOW_SYSTEM
+  /* Iconifying a frame might cause the frame title to change if no
+     title was explicitly specified.  Force the frame title to be
+     recomputed.  */
+
+  XSETFRAME (frame, f);
+
+  if (FRAME_WINDOW_P (f))
+    gui_consider_frame_title (frame);
+#endif
+}
 
 extern Lisp_Object selected_frame;
 extern Lisp_Object old_selected_frame;

src/xdisp.c

@@ -13148,7 +13148,7 @@ store_mode_line_noprop (const char *string, int field_width, int precision)
    Vicon_title_format if FRAME is iconified, otherwise it is
    frame_title_format.  */
 
-static void
+void
 gui_consider_frame_title (Lisp_Object frame)
 {
   struct frame *f = XFRAME (frame);