Add a couple of comments.

input.c

@@ -266,6 +266,7 @@ input_parse(struct window_pane *wp)
 
 	ictx->wp = wp;
 
+	/* If there is a mode set, don't want to update the screen. */
 	if (wp->mode == NULL)
 		screen_write_start(&ictx->ctx, wp, &wp->base);
 	else

tty.c

@@ -529,6 +529,7 @@ tty_write(void (*cmdfn)(
 	struct client		*c;
 	u_int		 	 i;
 
+	/* wp can be NULL if updating the screen but not the terminal. */
 	if (wp == NULL)
 		return;