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'.

3 files changed, 55 insertions, 2 deletions

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
@@ -5472,6 +5472,48 @@ 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:
  *
  * Return 1 if window W wants a header line and is high enough to
@@ -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.  */