Handle resizing of fullscreen frames more consistently (Bug#79704)

* src/frame.c (adjust_frame_size): Honor new option
'alter-fullscreen-frames'.
(syms_of_frame) <alter-fullscreen-frames>: New option to
maintain consistent state when attempting to resize fullscreen
frames.  Default to 'inhibit' for NS builds because these
resized the frame while leaving the 'fullscreen' parameter alone
(Bug#79704).
(syms_of_frame) <Qinhibit>: Define symbol.
* lisp/cus-start.el (standard): Add customization options for
'alter-fullscreen-frames'
* doc/lispref/frames.texi (Frame Size): Describe new option
'alter-fullscreen-frames'.
* etc/NEWS: Call out new option 'alter-fullscreen-frames'.

doc/lispref/frames.texi

@@ -1234,6 +1234,37 @@ bottom corner of the frame.  The parameters @code{min-width} and
 @code{min-height} (@pxref{Size Parameters}) can be used to obtain a
 similar behavior when changing the frame size from within Emacs.
 
+When a frame is in a fullscreen state (@pxref{Size Parameters}),
+requests to change the frame size via one of these functions may be
+supported or refused either by Emacs itself or by the window manager.
+The following variable provides more control of the resulting behavior.
+
+@cindex altering the size of fullscreen frames
+@cindex resizing fullscreen frames
+@defopt alter-fullscreen-frames
+This options controls how to handle requests to alter fullscreen frames.
+Emacs consults it when asked to resize a fullscreen frame via functions
+like @code{set-frame-size} or when setting the @code{width} or
+@code{height} parameter of a frame.  The following values are provided:
+
+@table @code
+@item nil
+This will forward the resize request to the window manager and leave it
+to the latter how to proceed.
+
+@item t
+This will first reset the fullscreen status and then forward the resize
+request on to the window manager.
+
+@item inhibit
+This will reject the resize request and leave the fullscreen status
+unchanged.
+@end table
+
+The default is @code{inhibit} on NS builds and @code{nil} everywhere
+else.
+@end defopt
+
 @cindex tracking frame size changes
   The abnormal hook @code{window-size-change-functions} (@pxref{Window
 Hooks}) tracks all changes of the inner size of a frame including those

etc/NEWS

@@ -444,6 +444,16 @@ This will inhibit implied resizing while a new frame is made and can be
 useful on tiling window managers where the initial frame size should be
 specified by external means.
 
++++
+*** New option 'alter-fullscreen-frames'.
+This option is useful to maintain a consistent state when attempting to
+resize fullscreen frames.  It defaults to 'inhibit' on NS builds which
+means that a fullscreen frame would not change size.  It defaults to nil
+everywhere else which means that the window manager is supposed to
+either resize the frame and change the fullscreen status accordingly or
+keep the frame size unchanged.  The value t means to first reset the
+fullscreen status and then resize the frame.
+
 ---
 *** Frames can now be renamed to F<number> on text terminals.
 Unlike with other frame names, an attempt to rename to F<number> throws

lisp/cus-start.el

@@ -347,6 +347,12 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
                             ;; :initialize custom-initialize-default
 			    :set custom-set-minor-mode)
 	     (frame-resize-pixelwise frames boolean "24.4")
+	     (alter-fullscreen-frames frames
+	                              (choice
+					    (const :tag "Forward request to window manager" nil)
+					    (const :tag "Reset fullscreen status first" t)
+					    (const :tag "Inhibit altering fullscreen frames" inhibit))
+				      "31.1")
 	     (frame-inhibit-implied-resize frames
 					   (choice
 					    (const :tag "Never" nil)

src/frame.c

@@ -786,6 +786,26 @@ adjust_frame_size (struct frame *f, int new_text_width, int new_text_height,
   min_inner_height
     = frame_windows_min_size (frame, Qnil, (inhibit == 5) ? Qsafe : Qnil, Qt);
 
+  if (inhibit == 1 && !NILP (alter_fullscreen_frames))
+    {
+      Lisp_Object fullscreen = get_frame_param (f, Qfullscreen);
+
+      if ((new_text_width != old_text_width
+	   && !NILP (fullscreen) && !EQ (fullscreen, Qfullheight))
+	  || (new_text_height != old_text_height
+	      && !NILP (fullscreen) && !EQ (fullscreen, Qfullwidth)))
+
+	{
+	  if (EQ (alter_fullscreen_frames, Qt))
+	    /* Reset fullscreen status and proceed.  */
+	    Fmodify_frame_parameters
+	      (frame, Fcons (Fcons (Qfullscreen, Qnil), Qnil));
+	  else if (EQ (alter_fullscreen_frames, Qinhibit))
+	    /* Do nothing and return.  */
+	    return;
+	}
+    }
+
   if (inhibit >= 2 && inhibit <= 4)
     /* When INHIBIT is in [2..4] inhibit if the "old" window sizes stay
        within the limits and either resizing is inhibited or INHIBIT
@@ -7102,6 +7122,7 @@ syms_of_frame (void)
   DEFSYM (Quse_frame_synchronization, "use-frame-synchronization");
   DEFSYM (Qfont_parameter, "font-parameter");
   DEFSYM (Qforce, "force");
+  DEFSYM (Qinhibit, "inhibit");
 
   for (int i = 0; i < ARRAYELTS (frame_parms); i++)
     {
@@ -7487,6 +7508,29 @@ allow `make-frame' to show the current buffer even if its hidden.  */);
   frame_internal_parameters = list3 (Qname, Qparent_id, Qwindow_id);
 #endif
 
+  DEFVAR_LISP ("alter-fullscreen-frames", alter_fullscreen_frames,
+	       doc: /* How to handle requests to alter fullscreen frames.
+Emacs consults this option when asked to resize a fullscreen frame via
+functions like 'set-frame-size' or when setting the 'width' or 'height'
+parameter of a frame.  The following values are provided:
+
+- nil means to forward the resize request to the window manager and
+  leave it to the latter how to proceed.
+
+- t means to first reset the fullscreen status and then forward the
+  request to the window manager.
+
+- 'inhibit' means to reject the resize request and leave the fullscreen
+  status unchanged.
+
+The default is 'inhibit' on NS builds and nil everywhere else.  */);
+
+#if defined (NS_IMPL_COCOA)
+  alter_fullscreen_frames = Qinhibit;
+#else
+  alter_fullscreen_frames = Qnil;
+#endif
+
   defsubr (&Sframep);
   defsubr (&Sframe_live_p);
   defsubr (&Swindow_system);