Fix bug with --no-number-headers when header is not at start of file.

Calculation in vlinenum was assuming header starts at position 0.
Related to #566.

NEWS

@@ -45,6 +45,9 @@
 
 * Fix bug when using +:n +:p +:x or +:d on the command line (github #552).
 
+* Fix bug with --no-number-headers when header is not at start of file
+  (github #566).
+
 ======================================================================
 
 	Major changes between "less" versions 661 and 664

linenum.c

@@ -70,6 +70,7 @@ extern int sigs;
 extern int sc_height;
 extern int header_lines;
 extern int nonum_headers;
+extern POSITION header_start_pos;
 
 /*
  * Initialize the line number structures.
@@ -511,7 +512,14 @@ public void scan_eof(void)
  */
 public LINENUM vlinenum(LINENUM linenum)
 {
-	if (nonum_headers)
-		linenum = (linenum < header_lines) ? 0 : linenum - header_lines;
+	if (nonum_headers && header_lines > 0)
+	{
+		LINENUM header_start_line = find_linenum(header_start_pos);
+		if (header_start_line != 0)
+		{
+			LINENUM header_end_line = header_start_line + header_lines; /* first line after header */
+			linenum = (linenum < header_end_line) ? 0 : linenum - header_end_line + 1;
+		}
+	}
 	return linenum;
 }

version.c

@@ -1030,7 +1030,9 @@ v663  8/16/24   Fix ^X bug when output is not a tty.
 v664            Fix Windows compile error.
 v659.1          Add --no-paste; add --no-edit-warn; add TAB for -- command;
                 add LESSANSIOSCALLOW and LESSANSIOSCCCHARS; fix bug searching 
-                near invalid UTF-8 sequence; add LESS_SHELL_LINES.
+                near invalid UTF-8 sequence; add LESS_SHELL_LINES; fix bug 
+                with --no-number-headers.
+
 */
 
 char version[] = "659.1x";