diff --git a/regress/window-copy-scrollbar-drag-focus-style.sh b/regress/window-copy-scrollbar-drag-focus-style.sh
new file mode 100755
index 000000000..77f478955
--- /dev/null
+++ b/regress/window-copy-scrollbar-drag-focus-style.sh
@@ -0,0 +1,123 @@
+#!/bin/sh
+
+# Dragging an inactive pane's scrollbar slider changes which pane is
+# active (window_copy_scroll(), window-copy.c). Every other place in the
+# codebase that changes the active pane pairs it with
+# window_redraw_active_switch() (or falls back to a full window redraw) so
+# the old/new active panes' window-active-style/window-style colours get
+# repainted - window_copy_scroll() was the one caller that did neither, so
+# a scrollbar-slider drag changed focus but left both panes' body colours
+# stale. This used to be masked by window_set_active_pane() itself doing
+# an unconditional full redraw on every active-pane change, until that was
+# narrowed to borders/status-only for the common case.
+#
+# This creates two tiled panes with clearly distinguishable
+# window-active-style/window-style backgrounds, puts the *inactive* one in
+# copy mode, drags its scrollbar slider, and checks - via an attached
+# client's own received bytes, since window-style is applied during
+# redraw composition rather than stored in the grid, so capture-pane
+# alone would not reflect it - that the newly active pane's marker text
+# immediately shows the active-style colour, not the stale one.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export PATH TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+
+DIR=$(mktemp -d) || exit 1
+INNER="$TEST_TMUX -Lsbfocus-inner-$$ -f/dev/null"
+OUTER="$TEST_TMUX -Lsbfocus-outer-$$ -f/dev/null"
+CAPTURE=$DIR/capture
+ESC=$(printf '\033')
+
+fail()
+{
+	echo "$*" >&2
+	[ -s "$CAPTURE" ] && cat -A "$CAPTURE" >&2
+	exit 1
+}
+
+cleanup()
+{
+	$OUTER kill-server 2>/dev/null
+	$INNER kill-server 2>/dev/null
+	rm -rf "$DIR"
+}
+trap cleanup 0 1 15
+
+get_mark_color()
+{
+	$OUTER capture-pane -e -p -t outer:0.0 >"$CAPTURE" 2>/dev/null || true
+	LC_ALL=C grep -a "MARK1" "$CAPTURE" |
+	    LC_ALL=C grep -aoE "${ESC}\[4[12]m" | tail -1
+}
+
+wait_mark_color()
+{
+	want=$1
+	i=0
+	while [ "$i" -lt 50 ]; do
+		[ "$(get_mark_color)" = "$want" ] && return 0
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "MARK1's pane never showed the expected background (wanted '$want', got '$(get_mark_color)')"
+}
+
+mouse()
+{
+	sequence=$(printf '\033[<%s;%s;%s%s' "$1" "$2" "$3" "$4")
+	$OUTER send-keys -t outer:0.0 -l "$sequence" || exit 1
+	sleep 0.1
+}
+
+$INNER new-session -d -s inner -x 40 -y 10 'sleep 100' || exit 1
+$INNER set-option -g status off || exit 1
+$INNER set-option -g window-size manual || exit 1
+$INNER set-option -g mouse on || exit 1
+$INNER set-option -g pane-scrollbars on || exit 1
+$INNER setw pane-scrollbars-position right || exit 1
+$INNER setw pane-scrollbars-style 'width=1,pad=0' || exit 1
+$INNER set-option -g window-active-style 'bg=red' || exit 1
+$INNER set-option -g window-style 'bg=green' || exit 1
+$INNER split-window -h -t inner -d 'printf MARK1; sleep 100' || exit 1
+
+PANE1=$($INNER list-panes -t inner -F '#{pane_id}' | sed -n 2p)
+[ "$($INNER display-message -p -t "$PANE1" '#{pane_active}')" = 0 ] ||
+	fail "sanity: MARK1's pane is already active before the drag"
+
+# Put the inactive pane in copy mode without switching focus (copy-mode -t
+# targets a pane without making it active).
+$INNER copy-mode -t "$PANE1" || exit 1
+
+$OUTER new-session -d -s outer -x 40 -y 10 'sleep 100' || exit 1
+$OUTER set-option -g status off || exit 1
+$OUTER set-option -g window-size manual || exit 1
+$OUTER set-option -g default-terminal screen-256color || exit 1
+$OUTER respawn-pane -k -t outer:0.0 \
+    "$TEST_TMUX -Lsbfocus-inner-$$ -f/dev/null attach-session -t inner" ||
+    exit 1
+
+wait_mark_color "${ESC}[42m"
+
+XOFF=$($INNER display-message -p -t "$PANE1" '#{pane_left}')
+YOFF=$($INNER display-message -p -t "$PANE1" '#{pane_top}')
+SX=$($INNER display-message -p -t "$PANE1" '#{pane_width}')
+SBCOL=$((XOFF + SX + 1))
+SBROW=$((YOFF + 1))
+
+# With no scrollback beyond the pane's own content, the slider fills the
+# whole scrollbar track, so any point on it (here, its very first row) is
+# on the slider.
+mouse 0 "$SBCOL" "$SBROW" M
+mouse 32 "$SBCOL" "$((SBROW + 1))" M
+mouse 0 "$SBCOL" "$((SBROW + 1))" m
+
+[ "$($INNER display-message -p -t "$PANE1" '#{pane_active}')" = 1 ] ||
+	fail "sanity: dragging the scrollbar slider did not change focus"
+
+wait_mark_color "${ESC}[41m"
+
+exit 0
diff --git a/window-copy.c b/window-copy.c
index fd42e91bd..8bfe5c07c 100644
--- a/window-copy.c
+++ b/window-copy.c
@@ -768,6 +768,7 @@ window_copy_scroll(struct window_pane *wp, int sl_mpos, u_int my,
 	struct window_mode_entry	*wme = TAILQ_FIRST(&wp->modes);
 
 	if (wme != NULL) {
+		window_redraw_active_switch(wp->window, wp);
 		window_set_active_pane(wp->window, wp, 0);
 		window_copy_scroll1(wme, wp, sl_mpos, my, tty_oy, scroll_exit);
 	}