diff --git a/cmd-refresh-client.c b/cmd-refresh-client.c
index da2a27976..11b43bf22 100644
--- a/cmd-refresh-client.c
+++ b/cmd-refresh-client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-refresh-client.c,v 1.54 2026/07/05 08:24:00 nicm Exp $ */
+/* $OpenBSD: cmd-refresh-client.c,v 1.55 2026/07/17 08:37:29 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -66,7 +66,6 @@ cmd_refresh_client_control_client_size(struct cmd *self, struct cmdq_item *item)
 	struct client		*tc = cmdq_get_target_client(item);
 	const char		*size = args_get(args, 'C');
 	u_int			 w, x, y;
-	struct client_window	*cw;
 
 	if (sscanf(size, "@%u:%ux%u", &w, &x, &y) == 3) {
 		if (x < WINDOW_MINIMUM || x > WINDOW_MAXIMUM ||
@@ -76,22 +75,16 @@ cmd_refresh_client_control_client_size(struct cmd *self, struct cmdq_item *item)
 		}
 		log_debug("%s: client %s window @%u: size %ux%u", __func__,
 		    tc->name, w, x, y);
-		cw = server_client_add_client_window(tc, w);
-		cw->sx = x;
-		cw->sy = y;
+		control_set_window_size(tc, w, x, y);
 		tc->flags |= CLIENT_WINDOWSIZECHANGED;
 		recalculate_sizes_now(1);
 		return (CMD_RETURN_NORMAL);
 	}
 	if (sscanf(size, "@%u:", &w) == 1) {
-		cw = server_client_get_client_window(tc, w);
-		if (cw != NULL) {
-			log_debug("%s: client %s window @%u: no size", __func__,
-			    tc->name, w);
-			cw->sx = 0;
-			cw->sy = 0;
-			recalculate_sizes_now(1);
-		}
+		log_debug("%s: client %s window @%u: no size", __func__,
+		    tc->name, w);
+		control_clear_window_size(tc, w);
+		recalculate_sizes_now(1);
 		return (CMD_RETURN_NORMAL);
 	}
 
diff --git a/control.c b/control.c
index 8b4ad3884..92645a8bc 100644
--- a/control.c
+++ b/control.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: control.c,v 1.61 2026/07/10 15:45:11 nicm Exp $ */
+/* $OpenBSD: control.c,v 1.62 2026/07/17 08:37:29 nicm Exp $ */
 
 /*
  * Copyright (c) 2012 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -78,9 +78,20 @@ struct control_pane {
 };
 RB_HEAD(control_panes, control_pane);
 
+/* Control client window size. */
+struct control_window {
+	u_int				 window;
+	u_int				 sx;
+	u_int				 sy;
+
+	RB_ENTRY(control_window)	 entry;
+};
+RB_HEAD(control_windows, control_window);
+
 /* Control client state. */
 struct control_state {
 	struct control_panes		 panes;
+	struct control_windows		 windows;
 
 	TAILQ_HEAD(, control_pane)	 pending_list;
 	u_int				 pending_count;
@@ -120,6 +131,18 @@ control_pane_cmp(struct control_pane *cp1, struct control_pane *cp2)
 }
 RB_GENERATE_STATIC(control_panes, control_pane, entry, control_pane_cmp);
 
+/* Compare control windows. */
+static int
+control_window_cmp(struct control_window *cw1, struct control_window *cw2)
+{
+	if (cw1->window < cw2->window)
+		return (-1);
+	if (cw1->window > cw2->window)
+		return (1);
+	return (0);
+}
+RB_GENERATE_STATIC(control_windows, control_window, entry, control_window_cmp);
+
 /* Free a block. */
 static void
 control_free_block(struct control_state *cs, struct control_block *cb)
@@ -161,6 +184,66 @@ control_add_pane(struct client *c, struct window_pane *wp)
 	return (cp);
 }
 
+/* Get window for this client. */
+static struct control_window *
+control_get_window(struct client *c, u_int window)
+{
+	struct control_state	*cs = c->control_state;
+	struct control_window	 cw = { .window = window };
+
+	if (cs == NULL)
+		return (NULL);
+	return (RB_FIND(control_windows, &cs->windows, &cw));
+}
+
+/* Set window size for this client. */
+void
+control_set_window_size(struct client *c, u_int window, u_int sx, u_int sy)
+{
+	struct control_state	*cs = c->control_state;
+	struct control_window	*cw;
+
+	if (cs == NULL)
+		return;
+	cw = control_get_window(c, window);
+	if (cw == NULL) {
+		cw = xcalloc(1, sizeof *cw);
+		cw->window = window;
+		RB_INSERT(control_windows, &cs->windows, cw);
+	}
+	cw->sx = sx;
+	cw->sy = sy;
+}
+
+/* Get window size for this client. */
+int
+control_get_window_size(struct client *c, u_int window, u_int *sx, u_int *sy)
+{
+	struct control_window	*cw;
+
+	if ((cw = control_get_window(c, window)) == NULL)
+		return (0);
+	*sx = cw->sx;
+	*sy = cw->sy;
+	return (1);
+}
+
+/* Clear window size for this client. */
+void
+control_clear_window_size(struct client *c, u_int window)
+{
+	struct control_state	*cs = c->control_state;
+	struct control_window	*cw;
+
+	if (cs == NULL)
+		return;
+	cw = control_get_window(c, window);
+	if (cw != NULL) {
+		RB_REMOVE(control_windows, &cs->windows, cw);
+		free(cw);
+	}
+}
+
 /* Discard output for a pane. */
 static void
 control_discard_pane(struct client *c, struct control_pane *cp)
@@ -749,6 +832,7 @@ control_start(struct client *c)
 
 	cs = c->control_state = xcalloc(1, sizeof *cs);
 	RB_INIT(&cs->panes);
+	RB_INIT(&cs->windows);
 	TAILQ_INIT(&cs->pending_list);
 	TAILQ_INIT(&cs->all_blocks);
 	cs->subs = monitor_create_client(c, control_sub_change, NULL);
@@ -800,6 +884,7 @@ control_stop(struct client *c)
 {
 	struct control_state	*cs = c->control_state;
 	struct control_block	*cb, *cb1;
+	struct control_window	*cw, *cw1;
 
 	if (cs == NULL)
 		return;
@@ -811,6 +896,10 @@ control_stop(struct client *c)
 	bufferevent_free(cs->read_event);
 
 	control_reset_offsets(c);
+	RB_FOREACH_SAFE(cw, control_windows, &cs->windows, cw1) {
+		RB_REMOVE(control_windows, &cs->windows, cw);
+		free(cw);
+	}
 	TAILQ_FOREACH_SAFE(cb, &cs->all_blocks, all_entry, cb1)
 		control_free_block(cs, cb);
 
diff --git a/resize.c b/resize.c
index 1d83e4b70..4f7ee490a 100644
--- a/resize.c
+++ b/resize.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: resize.c,v 1.57 2026/07/13 13:01:14 nicm Exp $ */
+/* $OpenBSD: resize.c,v 1.58 2026/07/17 08:37:29 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -134,9 +134,8 @@ clients_calculate_size(int type, int current, struct client *c,
     int, int, struct session *, struct window *), u_int *sx, u_int *sy,
     u_int *xpixel, u_int *ypixel)
 {
-	struct client		*loop;
-	struct client_window	*cw;
-	u_int			 cx, cy, n = 0;
+	struct client	*loop;
+	u_int		 cx, cy, n = 0;
 
 	/*
 	 * Start comparing with 0 for largest and UINT_MAX for smallest or
@@ -187,20 +186,10 @@ clients_calculate_size(int type, int current, struct client *c,
 			continue;
 		}
 
-		/*
-		 * If the client has a per-window size, use this instead if it is
-		 * smaller.
-		 */
-		if (w != NULL)
-			cw = server_client_get_client_window(loop, w->id);
-		else
-			cw = NULL;
-
 		/* Work out this client's size. */
-		if (cw != NULL && cw->sx != 0 && cw->sy != 0) {
-			cx = cw->sx;
-			cy = cw->sy;
-		} else {
+		if (w == NULL ||
+		    !control_get_window_size(loop, w->id, &cx, &cy) ||
+		    cx == 0 || cy == 0) {
 			cx = loop->tty.sx;
 			cy = loop->tty.sy - status_line_size(loop);
 		}
@@ -247,17 +236,16 @@ skip:
 			/* Look up per-window size if any. */
 			if (~loop->flags & CLIENT_WINDOWSIZECHANGED)
 				continue;
-			cw = server_client_get_client_window(loop, w->id);
-			if (cw == NULL)
+			if (!control_get_window_size(loop, w->id, &cx, &cy))
 				continue;
 
 			/* Clamp the size. */
 			log_debug("%s: %s size for @%u is %ux%u", __func__,
-			    loop->name, w->id, cw->sx, cw->sy);
-			if (cw->sx != 0 && *sx > cw->sx)
-				*sx = cw->sx;
-			if (cw->sy != 0 && *sy > cw->sy)
-				*sy = cw->sy;
+			    loop->name, w->id, cx, cy);
+			if (cx != 0 && *sx > cx)
+				*sx = cx;
+			if (cy != 0 && *sy > cy)
+				*sy = cy;
 		}
 	}
 	if (*sx != UINT_MAX && *sy != UINT_MAX)
diff --git a/server-client.c b/server-client.c
index e9c62a745..b4f39c1d7 100644
--- a/server-client.c
+++ b/server-client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: server-client.c,v 1.495 2026/07/17 08:13:23 nicm Exp $ */
+/* $OpenBSD: server-client.c,v 1.496 2026/07/17 08:37:29 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -23,6 +23,7 @@
 #include <errno.h>
 #include <event.h>
 #include <fcntl.h>
+#include <stdint.h>
 #include <imsg.h>
 #include <paths.h>
 #include <stdlib.h>
@@ -48,24 +49,46 @@ static void	server_client_set_path(struct client *);
 static void	server_client_set_progress_bar(struct client *);
 static void	server_client_reset_state(struct client *);
 static void	server_client_update_latest(struct client *);
+static void	server_client_remove_active_pane_client(struct client *);
 static void	server_client_dispatch(struct imsg *, void *);
 static int	server_client_dispatch_command(struct client *, struct imsg *);
 static int	server_client_dispatch_identify(struct client *, struct imsg *);
 static int	server_client_dispatch_shell(struct client *);
 static void	server_client_report_theme(struct client *, enum client_theme);
 
-/* Compare client windows. */
+/* Client active pane. */
+struct client_active_pane {
+	struct client		*client;
+	u_int			 window;
+	struct window_pane	*pane;
+
+	RB_ENTRY(client_active_pane)	 entry;
+};
+RB_HEAD(client_active_panes, client_active_pane);
+
+/* Compare client active panes. */
 static int
-server_client_window_cmp(struct client_window *cw1,
-    struct client_window *cw2)
+server_client_active_pane_cmp(struct client_active_pane *cap1,
+    struct client_active_pane *cap2)
 {
-	if (cw1->window < cw2->window)
+	uintptr_t	c1 = (uintptr_t)cap1->client;
+	uintptr_t	c2 = (uintptr_t)cap2->client;
+
+	if (c1 < c2)
 		return (-1);
-	if (cw1->window > cw2->window)
+	if (c1 > c2)
+		return (1);
+	if (cap1->window < cap2->window)
+		return (-1);
+	if (cap1->window > cap2->window)
 		return (1);
 	return (0);
 }
-RB_GENERATE(client_windows, client_window, entry, server_client_window_cmp);
+RB_GENERATE_STATIC(client_active_panes, client_active_pane, entry,
+    server_client_active_pane_cmp);
+
+static struct client_active_panes client_active_panes =
+    RB_INITIALIZER(&client_active_panes);
 
 /* Number of attached clients. */
 u_int
@@ -309,7 +332,6 @@ server_client_create(int fd)
 	c->out_fd = -1;
 
 	c->queue = cmdq_new();
-	RB_INIT(&c->windows);
 	RB_INIT(&c->files);
 
 	c->tty.sx = 80;
@@ -500,7 +522,6 @@ void
 server_client_lost(struct client *c)
 {
 	struct client_file	*cf, *cf1;
-	struct client_window	*cw, *cw1;
 
 	if (cfg_client == c)
 		cfg_client = NULL;
@@ -509,15 +530,12 @@ server_client_lost(struct client *c)
 	server_client_clear_overlay(c);
 	status_prompt_clear(c);
 	status_message_clear(c);
+	server_client_remove_active_pane_client(c);
 
 	RB_FOREACH_SAFE(cf, client_files, &c->files, cf1) {
 		cf->error = EINTR;
 		file_fire_done(cf);
 	}
-	RB_FOREACH_SAFE(cw, client_windows, &c->windows, cw1) {
-		RB_REMOVE(client_windows, &c->windows, cw);
-		free(cw);
-	}
 
 	TAILQ_REMOVE(&clients, c, entry);
 	log_debug("lost client %p", c);
@@ -3083,28 +3101,29 @@ server_client_get_flags(struct client *c)
 	return (s);
 }
 
-/* Get client window. */
-struct client_window *
-server_client_get_client_window(struct client *c, u_int id)
+/* Get client active pane. */
+static struct client_active_pane *
+server_client_get_active_pane(struct client *c, u_int id)
 {
-	struct client_window	cw = { .window = id };
+	struct client_active_pane	cap = { .client = c, .window = id };
 
-	return (RB_FIND(client_windows, &c->windows, &cw));
+	return (RB_FIND(client_active_panes, &client_active_panes, &cap));
 }
 
-/* Add client window. */
-struct client_window *
-server_client_add_client_window(struct client *c, u_int id)
+/* Add client active pane. */
+static struct client_active_pane *
+server_client_add_active_pane(struct client *c, u_int id)
 {
-	struct client_window	*cw;
+	struct client_active_pane	*cap;
 
-	cw = server_client_get_client_window(c, id);
-	if (cw == NULL) {
-		cw = xcalloc(1, sizeof *cw);
-		cw->window = id;
-		RB_INSERT(client_windows, &c->windows, cw);
+	cap = server_client_get_active_pane(c, id);
+	if (cap == NULL) {
+		cap = xcalloc(1, sizeof *cap);
+		cap->client = c;
+		cap->window = id;
+		RB_INSERT(client_active_panes, &client_active_panes, cap);
 	}
-	return (cw);
+	return (cap);
 }
 
 /* Get client active pane. */
@@ -3113,7 +3132,7 @@ server_client_get_pane(struct client *c)
 {
 	struct session		*s = c->session;
 	struct window		*w;
-	struct client_window	*cw;
+	struct client_active_pane	*cap;
 
 	if (s == NULL)
 		return (NULL);
@@ -3123,10 +3142,10 @@ server_client_get_pane(struct client *c)
 		return (w->modal);
 	if (~c->flags & CLIENT_ACTIVEPANE)
 		return (w->active);
-	cw = server_client_get_client_window(c, w->id);
-	if (cw == NULL)
+	cap = server_client_get_active_pane(c, w->id);
+	if (cap == NULL)
 		return (w->active);
-	return (cw->pane);
+	return (cap->pane);
 }
 
 /* Set client active pane. */
@@ -3134,32 +3153,47 @@ void
 server_client_set_pane(struct client *c, struct window_pane *wp)
 {
 	struct session		*s = c->session;
-	struct client_window	*cw;
+	struct client_active_pane	*cap;
 
 	if (s == NULL)
 		return;
 	if (wp->window->modal != NULL && wp != wp->window->modal)
 		return;
 
-	cw = server_client_add_client_window(c, s->curw->window->id);
-	cw->pane = wp;
+	cap = server_client_add_active_pane(c, s->curw->window->id);
+	cap->pane = wp;
 	log_debug("%s pane now %%%u", c->name, wp->id);
 }
 
-/* Remove pane from client lists. */
+/* Remove active pane entries for a client. */
+static void
+server_client_remove_active_pane_client(struct client *c)
+{
+	struct client_active_pane	*cap, *cap1;
+
+	RB_FOREACH_SAFE(cap, client_active_panes, &client_active_panes, cap1) {
+		if (cap->client == c) {
+			RB_REMOVE(client_active_panes, &client_active_panes, cap);
+			free(cap);
+		}
+	}
+}
+
+/* Remove pane from client state. */
 void
 server_client_remove_pane(struct window_pane *wp)
 {
-	struct client		*c;
-	struct window		*w = wp->window;
-	struct client_window	*cw;
+	struct client			*c;
+	struct client_active_pane	*cap, *cap1;
+
+	RB_FOREACH_SAFE(cap, client_active_panes, &client_active_panes, cap1) {
+		if (cap->pane == wp) {
+			RB_REMOVE(client_active_panes, &client_active_panes, cap);
+			free(cap);
+		}
+	}
 
 	TAILQ_FOREACH(c, &clients, entry) {
-		cw = server_client_get_client_window(c, w->id);
-		if (cw != NULL && cw->pane == wp) {
-			RB_REMOVE(client_windows, &c->windows, cw);
-			free(cw);
-		}
 		if (c->tty.mouse_last_pane == (int)wp->id) {
 			c->tty.mouse_last_pane = -1;
 			c->tty.mouse_drag_update = NULL;
diff --git a/tmux.h b/tmux.h
index 947401b4e..3d3cae5d8 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux.h,v 1.1406 2026/07/15 13:02:33 nicm Exp $ */
+/* $OpenBSD: tmux.h,v 1.1407 2026/07/17 08:37:29 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -2033,18 +2033,6 @@ struct client_file {
 };
 RB_HEAD(client_files, client_file);
 
-/* Client window. */
-struct client_window {
-	u_int			 window;
-	struct window_pane	*pane;
-
-	u_int			 sx;
-	u_int			 sy;
-
-	RB_ENTRY(client_window)	 entry;
-};
-RB_HEAD(client_windows, client_window);
-
 /* Maximum time to be pasting. */
 #define CLIENT_PASTE_TIME_LIMIT 5
 
@@ -2148,8 +2136,6 @@ struct client {
 	const char		*user;
 	struct cmdq_list	*queue;
 
-	struct client_windows	 windows;
-
 	struct control_state	*control_state;
 	u_int			 pause_age;
 
@@ -3237,7 +3223,6 @@ void printflike(1, 2) server_add_message(const char *, ...);
 int	 server_create_socket(uint64_t, char **);
 
 /* server-client.c */
-RB_PROTOTYPE(client_windows, client_window, entry, server_client_window_cmp);
 u_int	 server_client_how_many(void);
 void	 server_client_set_overlay(struct client *, u_int, overlay_check_cb,
 	     overlay_mode_cb, overlay_draw_cb, overlay_key_cb,
@@ -3265,8 +3250,6 @@ void	 server_client_loop(void);
 const char *server_client_get_cwd(struct client *, struct session *);
 void	 server_client_set_flags(struct client *, const char *);
 const char *server_client_get_flags(struct client *);
-struct client_window *server_client_get_client_window(struct client *, u_int);
-struct client_window *server_client_add_client_window(struct client *, u_int);
 struct window_pane *server_client_get_pane(struct client *);
 void	 server_client_set_pane(struct client *, struct window_pane *);
 void	 server_client_remove_pane(struct window_pane *);
@@ -3959,6 +3942,9 @@ void	control_set_pane_on(struct client *, struct window_pane *);
 void	control_set_pane_off(struct client *, struct window_pane *);
 void	control_continue_pane(struct client *, struct window_pane *);
 void	control_pause_pane(struct client *, struct window_pane *);
+void	control_set_window_size(struct client *, u_int, u_int, u_int);
+int	control_get_window_size(struct client *, u_int, u_int *, u_int *);
+void	control_clear_window_size(struct client *, u_int);
 struct window_pane_offset *control_pane_offset(struct client *,
 	   struct window_pane *, int *);
 void	control_reset_offsets(struct client *);