suspension: resume properly from an external SIGSTOP (when using Slang)

Slang apparently needs a call to SLsmg_refresh() to restore the screen
content and put the cursor in the right place.  But call this function
only when the suspension was actually caused by an external SIGSTOP,
because otherwise the original screen (from which nano was invoked)
gets plastered with nano's interface and content -- upon exit, this
is annoying and confusing.

Do not stuff a dummy keystroke into the input stream, as it seems to
get placed *after* the first byte of the next keystroke from the user.
That would cause an "Unknown sequence" for some keystrokes.

This fixes https://savannah.gnu.org/bugs/?59077.

Bug existed since version 2.8.5, commit 84ff9ebb.

src/nano.c

@@ -62,6 +62,10 @@ static struct termios original_state;
 
 static struct sigaction oldaction, newaction;
 		/* Containers for the original and the temporary handler for SIGINT. */
+#ifdef USE_SLANG
+static bool selfinduced = FALSE;
+		/* Whether a suspension was caused from inside nano or from outside. */
+#endif
 
 /* Create a new linestruct node.  Note that we do not set prevnode->next. */
 linestruct *make_new_node(linestruct *prevnode)
@@ -947,9 +951,15 @@ RETSIGTYPE do_suspend(int signal)
 /* Put nano to sleep (if suspension is enabled). */
 void do_suspend_void(void)
 {
-	if (ISSET(SUSPENDABLE))
+	if (ISSET(SUSPENDABLE)) {
+#ifdef USE_SLANG
+		selfinduced = TRUE;
 		do_suspend(0);
-	else {
+		selfinduced = FALSE;
+#else
+		do_suspend(0);
+#endif
+	} else {
 		statusbar(_("Suspension is not enabled"));
 		beep();
 	}
@@ -972,8 +982,13 @@ RETSIGTYPE do_continue(int signal)
 	/* Put the terminal in the desired state again. */
 	terminal_init();
 #endif
-	/* Tickle the input routine so it will update the screen. */
+#ifdef USE_SLANG
+	if (!selfinduced)
+		full_refresh();
+#else
+	/* Insert a fake keystroke, to neutralize a key-eating issue. */
 	ungetch(KEY_FLUSH);
+#endif
 }
 
 #if !defined(NANO_TINY) || defined(ENABLE_SPELLER) || defined(ENABLE_COLOR)