Add a regression test from Ilya Grigoriev.

regress/border-arrows.sh

@@ -0,0 +1,52 @@
+#!/bin/sh
+
+# Test for GitHub issue #4780 - pane-border-indicators both arrows missing
+# on second pane in a two-pane horizontal split.
+#
+# When pane-border-indicators is set to "both", arrow indicators should
+# appear when EITHER pane is selected. Before the fix, arrows only appeared
+# when the LEFT pane was selected.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Ltest"
+$TMUX kill-server 2>/dev/null
+TMUX_OUTER="$TEST_TMUX -Ltest2"
+$TMUX_OUTER kill-server 2>/dev/null
+
+trap "$TMUX kill-server 2>/dev/null; $TMUX_OUTER kill-server 2>/dev/null" 0 1 15
+
+# Start outer tmux that will capture the inner tmux's rendering
+$TMUX_OUTER -f/dev/null new -d -x80 -y24 "$TMUX -f/dev/null new -x78 -y22" || exit 1
+sleep 1
+
+# Set pane-border-indicators to "both" in inner tmux
+$TMUX set -g pane-border-indicators both || exit 1
+
+# Create horizontal split (two panes side by side)
+$TMUX splitw -h || exit 1
+sleep 1
+
+# Helper to check for arrow characters in captured output
+has_arrow() {
+    echo "$1" | grep -qE '(←|→|↑|↓)'
+}
+
+# Test 1: Select left pane (pane 0) and check for arrows
+$TMUX selectp -t 0
+sleep 1
+left_output=$($TMUX_OUTER capturep -Cep 2>/dev/null)
+has_arrow "$left_output" || exit 1
+
+# Test 2: Select right pane (pane 1) and check for arrows
+# This is the case that failed before the fix
+$TMUX selectp -t 1
+sleep 1
+right_output=$($TMUX_OUTER capturep -Cep 2>/dev/null)
+has_arrow "$right_output" || exit 1
+
+$TMUX kill-server 2>/dev/null
+$TMUX_OUTER kill-server 2>/dev/null
+exit 0