Add formats for pane written/skipped bytes for debugging.

format.c

@@ -2690,6 +2690,9 @@ format_defaults_pane(struct format_tree *ft, struct window_pane *wp)
 	format_add(ft, "history_limit", "%u", gd->hlimit);
 	format_add_cb(ft, "history_bytes", format_cb_history_bytes);
 
+	format_add(ft, "pane_written", "%zu", wp->written);
+	format_add(ft, "pane_skipped", "%zu", wp->skipped);
+
 	if (window_pane_index(wp, &idx) != 0)
 		fatalx("index not found");
 	format_add(ft, "pane_index", "%u", idx);

screen-write.c

@@ -174,6 +174,10 @@ screen_write_stop(struct screen_write_ctx *ctx)
 
 	log_debug("%s: %u cells (%u written, %u skipped)", __func__,
 	    ctx->cells, ctx->written, ctx->skipped);
+	if (ctx->wp != NULL) {
+		ctx->wp->written += ctx->written;
+		ctx->wp->skipped += ctx->skipped;
+	}
 
 	if (ctx->sync) {
 		screen_write_initctx(ctx, &ttyctx, 0);

tmux.1

@@ -4450,6 +4450,7 @@ The following variables are available, where appropriate:
 .It Li "pane_pipe" Ta "" Ta "1 if pane is being piped"
 .It Li "pane_right" Ta "" Ta "Right of pane"
 .It Li "pane_search_string" Ta "" Ta "Last search string in copy mode"
+.It Li "pane_skipped" Ta "" Ta "Bytes skipped as not visible in pane"
 .It Li "pane_start_command" Ta "" Ta "Command pane started with"
 .It Li "pane_synchronized" Ta "" Ta "1 if pane is synchronized"
 .It Li "pane_tabs" Ta "" Ta "Pane tab positions"
@@ -4457,6 +4458,7 @@ The following variables are available, where appropriate:
 .It Li "pane_top" Ta "" Ta "Top of pane"
 .It Li "pane_tty" Ta "" Ta "Pseudo terminal of pane"
 .It Li "pane_width" Ta "" Ta "Width of pane"
+.It Li "pane_written" Ta "" Ta "Bytes written by pane (aside from redrawing)"
 .It Li "pid" Ta "" Ta "Server PID"
 .It Li "popup_key" Ta "" Ta "Key pressed in popup"
 .It Li "popup_mouse_x" Ta "" Ta "Mouse X position in popup"

tmux.h

@@ -934,6 +934,9 @@ struct window_pane {
 	char		*searchstr;
 	int		 searchregex;
 
+	u_int		 written;
+	u_int		 skipped;
+
 	TAILQ_ENTRY(window_pane) entry;
 	RB_ENTRY(window_pane) tree_entry;
 };