Add support for focus notifications when tmux pane changes, based on work by

Aaron Jensen.

input.c

@@ -1260,6 +1260,9 @@ input_csi_dispatch(struct input_ctx *ictx)
 		case 1003:
 			screen_write_mode_clear(&ictx->ctx, ALL_MOUSE_MODES);
 			break;
+		case 1004:
+			screen_write_mode_clear(&ictx->ctx, MODE_FOCUSON);
+			break;
 		case 1005:
 			screen_write_mode_clear(&ictx->ctx, MODE_MOUSE_UTF8);
 			break;
@@ -1326,6 +1329,12 @@ input_csi_dispatch(struct input_ctx *ictx)
 			screen_write_mode_clear(&ictx->ctx, ALL_MOUSE_MODES);
 			screen_write_mode_set(&ictx->ctx, MODE_MOUSE_ANY);
 			break;
+		case 1004:
+			if (s->mode & MODE_FOCUSON)
+				break;
+			screen_write_mode_set(&ictx->ctx, MODE_FOCUSON);
+			wp->flags &= ~PANE_FOCUSED; /* force update if needed */
+			break;
 		case 1005:
 			screen_write_mode_set(&ictx->ctx, MODE_MOUSE_UTF8);
 			break;

server-client.c

@@ -27,6 +27,7 @@
 
 #include "tmux.h"
 
+void	server_client_check_focus(struct window_pane *);
 void	server_client_check_mouse(struct client *, struct window_pane *);
 void	server_client_repeat_timer(int, short, void *);
 void	server_client_check_exit(struct client *);
@@ -494,7 +495,7 @@ server_client_loop(void)
 
 	/*
 	 * Any windows will have been redrawn as part of clients, so clear
-	 * their flags now.
+	 * their flags now. Also check and update pane focus.
 	 */
 	for (i = 0; i < ARRAY_LENGTH(&windows); i++) {
 		w = ARRAY_ITEM(&windows, i);
@@ -502,11 +503,54 @@ server_client_loop(void)
 			continue;
 
 		w->flags &= ~WINDOW_REDRAW;
-		TAILQ_FOREACH(wp, &w->panes, entry)
+		TAILQ_FOREACH(wp, &w->panes, entry) {
+			server_client_check_focus(wp);
 			wp->flags &= ~PANE_REDRAW;
+		}
 	}
 }
 
+/* Check whether pane should be focused. */
+void
+server_client_check_focus(struct window_pane *wp)
+{
+	struct session	*s;
+
+	/* If we don't care about focus, forget it. */
+	if (!(wp->base.mode & MODE_FOCUSON))
+		return;
+
+	/* If we're not the active pane in our window, we're not focused. */
+	if (wp->window->active != wp)
+		goto not_focused;
+
+	/* If we're in a mode, we're not focused. */
+	if (wp->screen != &wp->base)
+		goto not_focused;
+
+	/*
+	 * If our window is the current window in any attached sessions, we're
+	 * focused.
+	 */
+	RB_FOREACH(s, sessions, &sessions) {
+		if (s->flags & SESSION_UNATTACHED)
+			continue;
+		if (s->curw->window == wp->window)
+			goto focused;
+	}
+
+not_focused:
+	if (wp->flags & PANE_FOCUSED)
+		bufferevent_write(wp->event, "\033[O", 3);
+	wp->flags &= ~PANE_FOCUSED;
+	return;
+
+focused:
+	if (!(wp->flags & PANE_FOCUSED))
+		bufferevent_write(wp->event, "\033[I", 3);
+	wp->flags |= PANE_FOCUSED;
+}
+
 /*
  * Update cursor position and mode settings. The scroll region and attributes
  * are cleared when idle (waiting for an event) as this is the most likely time

tmux.h

@@ -665,6 +665,7 @@ struct mode_key_table {
 #define MODE_MOUSE_UTF8 0x100
 #define MODE_MOUSE_SGR 0x200
 #define MODE_BRACKETPASTE 0x400
+#define MODE_FOCUSON 0x800
 
 #define ALL_MOUSE_MODES (MODE_MOUSE_STANDARD|MODE_MOUSE_BUTTON|MODE_MOUSE_ANY)
 
@@ -926,6 +927,7 @@ struct window_pane {
 	int		 flags;
 #define PANE_REDRAW 0x1
 #define PANE_DROP 0x2
+#define PANE_FOCUSED 0x4
 
 	char		*cmd;
 	char		*shell;

tty.c

@@ -221,7 +221,7 @@ tty_start_tty(struct tty *tty)
 		tty_puts(tty, "\033[?1000l\033[?1006l\033[?1005l");
 
 	if (tty_term_has(tty->term, TTYC_XT))
-		tty_puts(tty, "\033[c\033[>4;1m");
+		tty_puts(tty, "\033[c\033[>4;1m\033[?1004l");
 
 	tty->cx = UINT_MAX;
 	tty->cy = UINT_MAX;