diff --git a/regress/session-group-resize.sh b/regress/session-group-resize.sh
new file mode 100755
index 00000000..84eb31a4
--- /dev/null
+++ b/regress/session-group-resize.sh
@@ -0,0 +1,70 @@
+#!/bin/sh
+
+# Test that window-size=latest resizes windows correctly when switching
+# windows in session groups. When a client switches to a window, it should
+# resize immediately to match that client's size.
+#
+# Tests both switch-client and select-window, which use different code paths.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Ltest"
+$TMUX kill-server 2>/dev/null
+
+TMP1=$(mktemp)
+TMP2=$(mktemp)
+TMP3=$(mktemp)
+trap "rm -f $TMP1 $TMP2 $TMP3" 0 1 15
+
+# Create a session with two windows, staying on window 0.
+$TMUX -f/dev/null new -d -s test -x 20 -y 6 || exit 1
+$TMUX neww -t test || exit 1
+$TMUX selectw -t test:0 || exit 1
+
+# Attach a small 20x6 client in control-mode and have it select window 1. This makes
+# the small client the "latest" for window 1. The sleep keeps stdin open so the
+# control client stays attached.
+(echo "refresh-client -C 20,6"; echo "selectw -t :1"; sleep 5) |
+	$TMUX -f/dev/null -C attach -t test >$TMP1 2>&1 &
+
+# Wait for small client to be on window 1.
+n=0
+while [ $n -lt 20 ]; do
+	$TMUX lsc -F '#{client_name} #{window_index}' 2>/dev/null | grep -q " 1$" && break
+	sleep 0.1
+	n=$((n + 1))
+done
+
+# Create a grouped session with a larger 30x10 client, also in control mode. It
+# starts on window 0 (inherited), then switches to window 1 with
+# `switch-client`.
+(echo "refresh-client -C 30,10"; echo "switch-client -t :=1"; sleep 5) |
+	$TMUX -f/dev/null -C new -t test -x 30 -y 10 >$TMP2 2>&1 &
+
+# Wait briefly for the switch-client command to execute, then check.
+# The resize should happen immediately (within 0.2s).
+sleep 0.2
+OUT1=$($TMUX display -t test:1 -p '#{window_width}x#{window_height}' 2>/dev/null)
+
+# Also test selectw (select-window) which uses a different code path.
+# Create a third grouped session with a 25x8 client, switch to window 1
+# using selectw instead of switch-client.
+(echo "refresh-client -C 25,8"; echo "selectw -t :1"; sleep 5) |
+	$TMUX -f/dev/null -C new -t test -x 25 -y 8 >$TMP3 2>&1 &
+
+sleep 0.2
+OUT2=$($TMUX display -t test:1 -p '#{window_width}x#{window_height}' 2>/dev/null)
+
+# Clean up - kill server (terminates clients). Don't wait for background
+# sleeps; they'll be orphaned but harmless.
+$TMUX kill-server 2>/dev/null
+
+# Window 1 should have resized to 30x10 (the second client's size).
+[ "$OUT1" = "30x10" ] || { echo "switch-client resize failed: $OUT1"; exit 1; }
+
+# Window 1 should have resized to 25x8 (the third client's size).
+[ "$OUT2" = "25x8" ] || { echo "selectw resize failed: $OUT2"; exit 1; }
+
+exit 0