regress: add failing test for border-status redraw bug

window_make_pane_status() gates physical redraw of border-status text on
a logical content diff, not physical damage - if something else disturbs
those cells, the text stays blank because tmux believes nothing changed.
This test currently fails and is expected to keep failing until the
general damage-tracking redraw work lands (see IMAGE-REDRAW-PLAN.md).

regress/image-border-status-wipe.sh

@@ -0,0 +1,86 @@
+#!/bin/sh
+
+# Regression test for a KNOWN, NOT YET FIXED bug - this test currently
+# FAILS, and is expected to keep failing until the general damage-tracking
+# redraw work lands (IMAGE-REDRAW-PLAN.md, steps 2-4). Left failing
+# deliberately rather than skipped, so `make` in regress/ shows accurate,
+# live status of whether this is fixed yet.
+#
+# On a client detected as SIXEL-capable, entering copy-mode on a pane and
+# moving the cursor can blank a floating pane's border-status title text
+# (and can also disturb window-level border/title chrome), without
+# redrawing it back in. Root cause: window_make_pane_status()
+# (window-border.c) gates the physical redraw of border-status text on a
+# logical content diff (grid_compare against a cached copy), not on
+# whether the physical screen cells were disturbed by something else in
+# the meantime. See tmux-image-redraw-known-bugs.md ("border-status text
+# cache ignores physical damage") and IMAGE-REDRAW-DISCUSSION.md for the
+# full write-up.
+#
+# A proper damage-tracking compositor gates redraw on "was this region
+# touched", not "did the logical content change", so the disturbing erase
+# would itself register as damage and force recomposition regardless of
+# content diff. Design requirement for that work: border/status/scrollbar
+# spans must participate in the same damage-driven composition as pane
+# content, or this bug survives the rewrite unchanged.
+#
+# No mouse is required to reproduce this - entering copy-mode (matching a
+# PageUp key binding) and moving the cursor once is enough.
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export TERM LC_ALL
+
+CONF=$(mktemp)
+cat > "$CONF" <<EOF
+set -g mouse on
+set -g pane-scrollbars on
+set -g pane-border-status top
+set -g status 2
+bind-key -n PPage "copy-mode -u"
+bind-key -n NPage "copy-mode -d"
+EOF
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Libsw$$ -f/dev/null"
+TMUX2="$TEST_TMUX -Libsw-inner$$ -f $CONF"
+$TMUX kill-server 2>/dev/null
+$TMUX2 kill-server 2>/dev/null
+
+TMP=$(mktemp)
+trap "$TMUX kill-server 2>/dev/null; $TMUX2 kill-server 2>/dev/null; rm -f $TMP $CONF" 0 1 15
+
+$TMUX2 new-session -d -x 60 -y 20 "sh -c 'i=0; while [ \$i -lt 30 ]; do printf \"TILE%02d\\n\" \$i; i=\$((i + 1)); done; exec sleep 100'"
+$TMUX2 new-pane -x20 -y6 -X8 -Y3 -T FLOATTITLE "sh -c 'printf FLOATCONTENT; exec sleep 100'"
+[ "$($TMUX2 display-message -p '#{image_support}')" = 0 ] && exit 0
+$TMUX2 set -as terminal-features ',*:sixel'
+
+$TMUX new-session -d -x 60 -y 20
+$TMUX send -l "$TMUX2 attach"
+$TMUX send Enter
+sleep 1
+
+# Sanity check: the floating pane's OWN border row shows its title before
+# anything disturbs it. (A bare 'FLOATTITLE' grep on the whole capture is
+# not enough - the outer client's title bar separately reflects the pane
+# title via an unrelated OSC title escape, regardless of this bug.)
+$TMUX capturep -p >$TMP || exit 1
+grep '┌' $TMP | grep -q 'FLOATTITLE' || exit 1
+
+# Enter copy-mode on the (active) floating pane, matching the PageUp
+# binding, then move the cursor once, matching the Up binding.
+$TMUX2 copy-mode -u -t%1
+sleep 1
+$TMUX2 send-keys -t%1 -X cursor-up
+sleep 1
+
+$TMUX capturep -p >$TMP || exit 1
+
+# The border frame and its title text should both still be there. This is
+# expected to fail today - see the header comment.
+BORDERLINE=$(grep '┌' $TMP)
+[ -n "$BORDERLINE" ] || exit 1
+echo "$BORDERLINE" | grep -q 'FLOATTITLE' || exit 1
+
+exit 0