scrolling: add a function and a key binding to center the cursor line

By default, this function is bound to ^L, to make that keystroke do
something actually useful.  To not lose the Refresh function that this
keystroke had, the centering function additionally does a full redraw
and refresh of the screen.

doc/nano.texi

@@ -1370,6 +1370,9 @@ while keeping the cursor in the same text position, if possible.
 Scrolls the viewport down one row (meaning that the text slides up)
 while keeping the cursor in the same text position, if possible.
 
+@item center
+Scrolls the line with the cursor to the middle of the screen.
+
 @item prevword
 Moves the cursor to the beginning of the previous word.
 

doc/nanorc.5

@@ -674,6 +674,9 @@ while keeping the cursor in the same text position, if possible.
 Scrolls the viewport down one row (meaning that the text slides up)
 while keeping the cursor in the same text position, if possible.
 .TP
+.B center
+Scrolls the line with the cursor to the middle of the screen.
+.TP
 .B prevword
 Moves the cursor to the beginning of the previous word.
 .TP

src/global.c

@@ -622,6 +622,7 @@ void shortcut_init(void)
 		N_("Scroll up one line without moving the cursor textually");
 	const char *scrolldown_gist =
 		N_("Scroll down one line without moving the cursor textually");
+	const char *center_gist = N_("Center the line where the cursor is");
 #endif
 #ifdef ENABLE_MULTIBUFFER
 	const char *prevfile_gist = N_("Switch to the previous file buffer");
@@ -1035,7 +1036,11 @@ void shortcut_init(void)
 				N_("Formatter"), WITHORSANS(formatter_gist), BLANKAFTER, NOVIEW);
 #endif
 	}
-#endif /* NANO_TINY */
+#endif /* !NANO_TINY */
+#if !defined(NANO_TINY) || defined(ENABLE_HELP)
+	add_to_funcs(do_center, MMAIN,
+		N_("Center"), WITHORSANS(center_gist), BLANKAFTER, VIEW);
+#endif
 	add_to_funcs(do_savefile, MMAIN,
 		N_("Save"), WITHORSANS(savefile_gist), BLANKAFTER, NOVIEW);
 
@@ -1322,6 +1327,9 @@ void shortcut_init(void)
 #endif
 #ifdef ENABLE_JUSTIFY
 	add_to_sclist(MEXECUTE, "^J", 0, do_full_justify, 0);
+#endif
+#if !defined(NANO_TINY) || defined(ENABLE_HELP)
+	add_to_sclist(MMAIN, "^L", 0, do_center, 0);
 #endif
 	if (!ISSET(PRESERVE))
 		add_to_sclist(MMOST|MBROWSER|MHELP|MYESNO, "^L", 0, full_refresh, 0);

src/move.c

@@ -562,7 +562,15 @@ void do_scroll_down(void)
 										))
 		edit_scroll(FORWARD);
 }
-#endif
+
+/* Scroll the line with the cursor to the center of the screen. */
+void do_center(void)
+{
+	adjust_viewport(CENTERING);
+	draw_all_subwindows();
+	full_refresh();
+}
+#endif /* !NANO_TINY || ENABLE_HELP */
 
 /* Move left one character. */
 void do_left(void)

src/prototypes.h

@@ -386,6 +386,7 @@ void do_down(void);
 #if !defined(NANO_TINY) || defined(ENABLE_HELP)
 void do_scroll_up(void);
 void do_scroll_down(void);
+void do_center(void);
 #endif
 void do_left(void);
 void do_right(void);

src/rcfile.c

@@ -348,6 +348,8 @@ keystruct *strtosc(const char *input)
 		s->func = do_scroll_up;
 	else if (!strcmp(input, "scrolldown"))
 		s->func = do_scroll_down;
+	else if (!strcmp(input, "center"))
+		s->func = do_center;
 #endif
 	else if (!strcmp(input, "prevword"))
 		s->func = to_prev_word;