diff --git a/regress/border-zoom-click.sh b/regress/border-zoom-click.sh
new file mode 100644
index 000000000..5b22902bf
--- /dev/null
+++ b/regress/border-zoom-click.sh
@@ -0,0 +1,51 @@
+#!/bin/sh
+
+# The zoom button on a pane's border must zoom the pane that was clicked, not
+# the active pane.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f/dev/null"
+
+cleanup()
+{
+	$TMUX kill-server >/dev/null 2>&1
+	$TMUX2 kill-server >/dev/null 2>&1
+}
+fail()
+{
+	echo "$*" >&2
+	cleanup
+	exit 1
+}
+
+cleanup
+
+$TMUX new-session -d -s inner -x 80 -y 24 'sleep 100' || exit 1
+$TMUX set -g mouse on || exit 1
+$TMUX set -g pane-border-status top || exit 1
+$TMUX split-window -d -h 'sleep 100' || exit 1
+active=$($TMUX display-message -p '#{pane_id}') || exit 1
+other=$($TMUX list-panes -F '#{pane_id}' | grep -v "^$active\$") || exit 1
+
+$TMUX2 new-session -d -x 80 -y 24 "$TMUX attach -t inner" || exit 1
+sleep 1
+OUTER=$($TMUX2 list-panes -F '#{pane_id}' | head -1)
+[ -n "$OUTER" ] || fail "No outer pane."
+
+# The right hand pane's border ends with the buttons [f][z][x]; the zoom one is
+# the middle button, at column 76 (1-based) on the first row.
+seq=$(printf '\033[<0;76;1M\033[<0;76;1m')
+$TMUX2 send-keys -t "$OUTER" -l "$seq" || fail "send-keys failed."
+sleep 1
+
+got=$($TMUX display-message -p -t "$other" '#{pane_zoomed_flag}')
+[ "$got" = "1" ] || fail "clicked pane zoomed flag '$got', expected '1'"
+got=$($TMUX display-message -p -t "$active" '#{pane_zoomed_flag}')
+[ "$got" = "0" ] || fail "active pane zoomed flag '$got', expected '0'"
+
+cleanup
+exit 0
diff --git a/regress/floating-pane-zoom-stack.sh b/regress/floating-pane-zoom-stack.sh
new file mode 100644
index 000000000..0068cc6c4
--- /dev/null
+++ b/regress/floating-pane-zoom-stack.sh
@@ -0,0 +1,248 @@
+#!/bin/sh
+
+# Zoom is a state of a pane inside a single stacking order rather than a
+# window-wide layout swap. Any pane, tiled or floating, may be zoomed, several
+# panes may be zoomed at once, and floats may sit above a zoomed pane.
+#
+# This exercises:
+# - creating a float while a pane is zoomed leaving that zoom alone;
+# - resize-pane -Z toggling only the target pane and resize-pane -a -Z clearing
+#   every zoom;
+# - activating a covered float raising it, and activating a tiled pane that is
+#   not zoomed unzooming everything but leaving the floats floating;
+# - pane-raise-on-focus deciding whether activating a visible zoomed pane
+#   raises it;
+# - new-pane -A staying above everything raised later;
+# - commands that only change the tiled layout leaving zooms alone, and
+#   select-layout/next-layout still unzooming;
+# - break-pane -W, move-pane and kill-pane working while panes are zoomed.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+$TMUX kill-server 2>/dev/null
+trap '$TMUX kill-server 2>/dev/null' 0
+trap 'exit 1' 1 2 15
+
+fail()
+{
+	echo "$*" >&2
+	exit 1
+}
+
+run()
+{
+	$TMUX "$@" || fail "failed: $*"
+}
+
+# check $target $format $expected
+check()
+{
+	got=$(run display-message -p -t "$1" "$2") || exit 1
+	[ "$got" = "$3" ] || fail "$1 $2: got '$got', expected '$3'"
+}
+
+# zoomed $pane $expected: is the pane itself zoomed?
+zoomed()
+{
+	check "$1" '#{pane_zoomed_flag}' "$2"
+}
+
+# window_zoomed $pane $expected: is any pane in the window zoomed?
+window_zoomed()
+{
+	check "$1" '#{window_zoomed_flag}' "$2"
+}
+
+# front $pane: is the pane at the front of the stacking order?
+front()
+{
+	check "$1" '#{pane_z}' 0
+}
+
+# ahead $pane1 $pane2: is the first pane above the second in the stack?
+ahead()
+{
+	z1=$(run display-message -p -t "$1" '#{pane_z}') || exit 1
+	z2=$(run display-message -p -t "$2" '#{pane_z}') || exit 1
+	[ "$z1" -lt "$z2" ] || fail "$1 (z=$z1) not above $2 (z=$z2)"
+}
+
+reset()
+{
+	$TMUX kill-server 2>/dev/null
+	run new-session -d -x 80 -y 24
+	A=$(run display-message -p '#{pane_id}') || exit 1
+	B=$(run split-window -dPF '#{pane_id}') || exit 1
+	layout=$(run display-message -p '#{window_layout}') || exit 1
+}
+
+# Creating a float while a pane is zoomed must not unzoom it.
+reset
+run resize-pane -Z -t "$A"
+window_zoomed "$A" 1
+X=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+zoomed "$A" 1
+zoomed "$X" 0
+check "$X" '#{pane_floating_flag}:#{pane_active}' '1:1'
+ahead "$X" "$A"
+window_zoomed "$A" 1
+
+# Zooming a second pane stacks it above the first; toggling it leaves the
+# first zoomed (this is the failing case from GitHub issue 5135).
+Y=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 40 -Y 6 '') || exit 1
+run resize-pane -Z -t "$Y"
+zoomed "$Y" 1
+zoomed "$A" 1
+window_zoomed "$A" 1
+check "$Y" '#{pane_width}:#{pane_height}' '80:24'
+run resize-pane -Z -t "$Y"
+zoomed "$Y" 0
+zoomed "$A" 1
+window_zoomed "$A" 1
+check "$Y" '#{pane_floating_flag}' 1
+
+# A zoomed float is still a float.
+run resize-pane -Z -t "$Y"
+check "$Y" '#{pane_floating_flag}:#{pane_zoomed_flag}' '1:1'
+run resize-pane -Z -t "$Y"
+
+# Unzooming the bottom zoom leaves the floats where they are.
+run resize-pane -Z -t "$A"
+zoomed "$A" 0
+window_zoomed "$A" 0
+check "$X" '#{pane_floating_flag}' 1
+check "$Y" '#{pane_floating_flag}' 1
+check "$A" '#{pane_width}:#{pane_height}' '80:12'
+
+# resize-pane -a -Z clears every zoom.
+run resize-pane -Z -t "$A"
+run resize-pane -Z -t "$B"
+run resize-pane -Z -t "$X"
+zoomed "$A" 1
+zoomed "$B" 1
+zoomed "$X" 1
+run resize-pane -a -Z -t "$A"
+for p in "$A" "$B" "$X" "$Y"; do
+	zoomed "$p" 0
+done
+window_zoomed "$A" 0
+
+# Activating a float that is covered by another zoomed pane raises it and
+# leaves every zoom alone.
+reset
+run resize-pane -Z -t "$A"
+X=$(run new-pane -dPF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+Y=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 40 -Y 6 '') || exit 1
+run resize-pane -Z -t "$Y"
+run select-pane -t "$X"
+check "$X" '#{pane_active}' 1
+front "$X"
+zoomed "$Y" 1
+zoomed "$A" 1
+window_zoomed "$X" 1
+
+# Activating a tiled pane that is not zoomed unzooms every pane but the
+# floats stay floats.
+run select-pane -t "$B"
+check "$B" '#{pane_active}' 1
+for p in "$A" "$B" "$X" "$Y"; do
+	zoomed "$p" 0
+done
+window_zoomed "$B" 0
+check "$X" '#{pane_floating_flag}' 1
+check "$Y" '#{pane_floating_flag}' 1
+
+# Activating a tiled pane that is itself zoomed does not unzoom it.
+run resize-pane -Z -t "$A"
+run select-pane -t "$A"
+zoomed "$A" 1
+window_zoomed "$A" 1
+run resize-pane -a -Z -t "$A"
+
+# pane-raise-on-focus: by default only floats are raised.
+reset
+run resize-pane -Z -t "$A"
+X=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+Y=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 40 -Y 6 '') || exit 1
+front "$Y"
+run select-pane -t "$A"
+front "$Y"
+ahead "$X" "$A"
+run select-pane -t "$X"
+front "$X"
+run set -w -t "$A" pane-raise-on-focus off
+run select-pane -t "$Y"
+ahead "$X" "$Y"
+run set -w -t "$A" pane-raise-on-focus all
+run select-pane -t "$A"
+front "$A"
+run select-pane -t "$Y"
+front "$Y"
+run set -wu -t "$A" pane-raise-on-focus
+
+# new-pane -A stays above everything raised later.
+reset
+run resize-pane -Z -t "$A"
+X=$(run new-pane -APF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+Y=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 40 -Y 6 '') || exit 1
+run resize-pane -Z -t "$Y"
+ahead "$X" "$Y"
+run select-pane -t "$Y"
+ahead "$X" "$Y"
+zoomed "$A" 1
+
+# Losing the active pane goes to the last-used pane when it is visible, and
+# does not unzoom.
+reset
+run resize-pane -Z -t "$A"
+X=$(run new-pane -PF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+run kill-pane -t "$X"
+check "$A" '#{pane_active}' 1
+window_zoomed "$A" 1
+
+# Commands that only change the tiled layout leave the zoom alone.
+run split-window -d -t "$B"
+C=$(run display-message -p -t "$B" '#{pane_id}') || exit 1
+zoomed "$A" 1
+window_zoomed "$A" 1
+run kill-pane -t "$B"
+zoomed "$A" 1
+window_zoomed "$A" 1
+
+# ... but select-layout and next-layout still unzoom.
+run next-layout -t "$A"
+window_zoomed "$A" 0
+run resize-pane -Z -t "$A"
+run select-layout -t "$A" even-horizontal
+window_zoomed "$A" 0
+
+# A zoomed pane can be floated and a float can be zoomed and tiled again.
+reset
+run resize-pane -Z -t "$B"
+run break-pane -W -s "$B"
+check "$B" '#{pane_floating_flag}:#{pane_zoomed_flag}' '1:1'
+run join-pane -s "$B" -t "$B"
+check "$B" '#{pane_floating_flag}:#{pane_zoomed_flag}' '0:1'
+run resize-pane -Z -t "$B"
+
+# A float covered by a zoomed pane can still be moved and resized without
+# unzooming anything.
+reset
+X=$(run new-pane -dPF '#{pane_id}' -t "$A" -x 20 -y 8 -X 8 -Y 3 '') || exit 1
+run resize-pane -Z -t "$A"
+run move-pane -t "$X" -X 20 -Y 5
+check "$X" '#{pane_left}:#{pane_top}' '21:6'
+run resize-pane -t "$X" -x 30
+check "$X" '#{pane_width}' 28
+zoomed "$A" 1
+window_zoomed "$A" 1
+
+# The window can be resized with panes zoomed and the zooms follow.
+run resize-window -t "$A" -x 100 -y 30
+check "$A" '#{pane_width}:#{pane_height}' '100:30'
+zoomed "$A" 1
+
+exit 0