master 4f66cbbfe52 1/2: Avoid header line with some empty non-nil formats

[Eli Zaretskii]

branch: master
commit 4f66cbbfe520ee31ef26676e09a926217d9736fe
Author: Eshel Yaron <eshelyaron@dazzs-mbp.home>
Commit: Eli Zaretskii <eliz@gnu.org>

    Avoid header line with some empty non-nil formats
    
    Allow the value of 'header-line-format' to indicate that no header
    line should be displayed when it trivially yields 'nil', even if it is
    not plain 'nil'.  Previously, any non-nil 'header-line-format'
    resulted in a (possibly empty) header line.  This change adds some
    flexibility by also taking a non-nil value of 'header-line-format' to
    mean that no header line should be displayed if it's a list whose
    'car' is a symbol and either that symbol is ':eval' and the second
    list element evaluates to 'nil', or the symbol's value as a variable
    is 'nil' or void.
    (Bug#63825)
    
    * src/xdisp.c (safe_eval_inhibit_quit): New function.
    * src/lisp.h (safe_eval_inhibit_quit): Declare it.
    * src/window.c (null_header_line_format): New function.
    (window_wants_header_line): Use it.
    
    * doc/lispref/modes.texi (Header Line): Update to reflect new
    conditions for displaying a window's header line.
    
    * etc/NEWS: Announce updated treatment of 'header-line-format'.
---
 doc/lispref/modes.texi |  9 +++++++++
 etc/NEWS               |  8 ++++++++
 src/lisp.h             |  1 +
 src/window.c           | 50 ++++++++++++++++++++++++++++++++++++++++++++++++--
 src/xdisp.c            |  6 ++++++
 5 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index c2698da6d99..d2a05fe29e4 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -2597,6 +2597,15 @@ It is normally @code{nil}, so that ordinary buffers have 
no header
 line.
 @end defvar
 
+Emacs displays the header line for a window unless
+@code{header-line-format} is either @code{nil}, or it's a list whose
+@sc{car} is a symbol, and either that symbol is @code{:eval} and the
+second list element evaluates to @code{nil} or the symbol's value as a
+variable is @code{nil} or void.  Note that there are other possible
+values @code{header-line-format} that result in an empty header line
+(for example, @code{""}), but all other values tell Emacs to display a
+header line, whether or not it is empty.
+
 If @code{display-line-numbers-mode} is turned on in a buffer
 (@pxref{Display Custom, display-line-numbers-mode,, emacs, The GNU
 Emacs Manual}), the buffer text is indented on display by the amount
diff --git a/etc/NEWS b/etc/NEWS
index 910472e5519..ec1b40aa0a7 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -451,6 +451,14 @@ hooks named after the feature name, like 
'esh-mode-unload-hook'.
 +++
 ** 'copy-tree' now copies records when its optional 2nd argument is non-nil.
 
++++
+** Certain values of 'header-line-format' now inhibit empty header line.
+Emacs now avoids displaying a header line, instead of displaying an
+empty one, when 'header-line-format' is a list whose 'car' is a
+symbol, and either that symbol is ':eval' and the second element of
+the list evaluates to 'nil' or the symbol's value as a variable is
+'nil' or void.
+
 
 * Lisp Changes in Emacs 30.1
 
diff --git a/src/lisp.h b/src/lisp.h
index 2bfcd1a1983..2978de962d9 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4174,6 +4174,7 @@ void set_frame_cursor_types (struct frame *, Lisp_Object);
 extern void syms_of_xdisp (void);
 extern void init_xdisp (void);
 extern Lisp_Object safe_eval (Lisp_Object);
+extern Lisp_Object safe_eval_inhibit_quit (Lisp_Object);
 extern bool pos_visible_p (struct window *, ptrdiff_t, int *,
                           int *, int *, int *, int *, int *);
 
diff --git a/src/window.c b/src/window.c
index f4e09f49eae..9429679061e 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5471,6 +5471,48 @@ window_wants_mode_line (struct window *w)
 }
 
 
+/**
+ * null_header_line_format:
+ *
+ * Return non-zero when header line format FMT indicates that the
+ * header line should not be displayed at all.
+ *
+ * This is when FMT is nil, or if FMT is a cons cell and either its
+ * car is a symbol whose value as a variable is nil or void, or its
+ * car is the symbol ':eval' and its cadr evaluates to nil.
+ */
+static bool
+null_header_line_format (Lisp_Object fmt, struct frame * f)
+{
+  Lisp_Object car;
+  Lisp_Object val;
+
+  if (NILP (fmt))
+    return true;
+
+  if (CONSP (fmt))
+    {
+      car = XCAR (fmt);
+      if (SYMBOLP (car))
+       {
+         if (EQ (car, QCeval))
+           {
+             val = safe_eval_inhibit_quit (XCAR (XCDR (fmt)));
+             if (!FRAME_LIVE_P (f))
+               signal_error (":eval deleted the frame being displayed", fmt);
+             return NILP (val);
+           }
+         val = find_symbol_value (car);
+         return (SYMBOLP (car)
+                 && (EQ (val, Qunbound)
+                     || NILP (val)));
+       }
+    }
+
+  return false;
+}
+
+
 /**
  * window_wants_header_line:
  *
@@ -5491,12 +5533,16 @@ window_wants_header_line (struct window *w)
   Lisp_Object window_header_line_format =
     window_parameter (w, Qheader_line_format);
 
+  struct frame * f = WINDOW_XFRAME(w);
+
   return (WINDOW_LEAF_P (w)
          && !MINI_WINDOW_P (w)
          && !WINDOW_PSEUDO_P (w)
          && !EQ (window_header_line_format, Qnone)
-         && (!NILP (window_header_line_format)
-             || !NILP (BVAR (XBUFFER (WINDOW_BUFFER (w)), header_line_format)))
+         && (!null_header_line_format (window_header_line_format, f)
+             || !null_header_line_format (BVAR (XBUFFER (WINDOW_BUFFER (w)),
+                                                header_line_format),
+                                          f))
          && (WINDOW_PIXEL_HEIGHT (w)
              > (window_wants_mode_line (w)
                 ? 2 * WINDOW_FRAME_LINE_HEIGHT (w)
diff --git a/src/xdisp.c b/src/xdisp.c
index a6ec966ea3c..5e25857322f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3074,6 +3074,12 @@ safe__eval (bool inhibit_quit, Lisp_Object sexpr)
   return safe__call1 (inhibit_quit, Qeval, sexpr);
 }
 
+Lisp_Object
+safe_eval_inhibit_quit (Lisp_Object sexpr)
+{
+  return safe__eval (true, sexpr);
+}
+
 /* Call function FN with two arguments ARG1 and ARG2.
    Return the result, or nil if something went wrong.  */