server-client: don't let a drag's own sync-start defer its redraw

server_client_key_callback()'s mouse-drag dispatch opens a
synchronized-output frame (tty_sync_start()) before running the drag
callback, on every single drag motion event - deliberately, so a fast-
path write and any later damage-composed correction land in one atomic
terminal update instead of two visible frames. But
server_client_check_redraw() later in the same pass checks
EVBUFFER_LENGTH(tty->out) != 0 to decide whether to defer this pass's
redraw, and nothing drains tty->out in between (the actual write
happens later, via libevent) - so the frame-open sequence just queued
(8 bytes: "\033[?2026h" on a synchronized-output-capable terminal)
makes that check see "outstanding output" and defer against itself,
escalating the drag's damage to a full-window redraw on every single
motion event. Confirmed via the server's own -vv log: a 6-step drag
produced five "redraw deferred (8 left)" lines, one per motion event,
with no fix.

Fix: record how much was already queued at the instant the sync frame
opened (tty->sync_offset), and have the redraw check discount
anything queued after that point - it's already part of the frame
this pass is committed to flushing, not a reason to defer. If the
buffer was genuinely non-empty before the frame opened, sync_offset
holds that real backlog and deferral still happens correctly.

regress/floating-pane-drag-sync-no-self-defer.sh drags a floating pane
on a synchronized-output-capable terminal and asserts the server log
never shows the self-inflicted 8-byte deferral. Verified failing 3/3
against the pre-fix code (5 occurrences per run, matching the manual
-vv repro) and passing 5/5 against the fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

regress/floating-pane-drag-sync-no-self-defer.sh

@@ -0,0 +1,95 @@
+#!/bin/sh
+
+# server_client_key_callback()'s mouse-drag dispatch opens a synchronized-
+# output frame (tty_sync_start()) before running the drag callback, on
+# every single drag motion event. server_client_check_redraw() then checks
+# EVBUFFER_LENGTH(tty->out) != 0 later in the same pass to decide whether
+# to defer this pass's redraw - nothing drains tty->out in between, so the
+# frame-open sequence just queued (8 bytes: "\033[?2026h") makes that check
+# see "outstanding output" and defer against itself, escalating the drag's
+# damage to a full-window redraw on every motion event on any
+# synchronized-output-capable terminal. This checks the server's own -vv
+# log for that exact self-inflicted "8 left" deferral pattern during a
+# drag, and requires it never appears.
+
+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=$(mktemp -d) || exit 1
+cd "$DIR" || exit 1
+INNER="$TEST_TMUX -vv -Lsyncdefer-inner-$$ -f/dev/null"
+OUTER="$TEST_TMUX -Lsyncdefer-outer-$$ -f/dev/null"
+
+fail()
+{
+	echo "$*" >&2
+	exit 1
+}
+
+cleanup()
+{
+	$OUTER kill-server 2>/dev/null
+	$INNER kill-server 2>/dev/null
+	cd /
+	rm -rf "$DIR"
+}
+trap cleanup 0 1 15
+
+mouse()
+{
+	sequence=$(printf '\033[<%s;%s;%s%s' "$1" "$2" "$3" "$4")
+	$OUTER send-keys -t outer:0.0 -l "$sequence" || exit 1
+	sleep 0.15
+}
+
+$INNER new-session -d -s inner -x 40 -y 10 'sleep 100' || exit 1
+$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
+FLOAT=$($INNER new-pane -d -PF '#{pane_id}' -x 15 -y 5 -X 5 -Y 2 \
+    'sleep 100') || 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
+$OUTER set-option -as terminal-features ',screen-256color:sync' || exit 1
+$OUTER respawn-pane -k -t outer:0.0 \
+    "$TEST_TMUX -Lsyncdefer-inner-$$ -f/dev/null attach-session -t inner" ||
+    exit 1
+sleep 0.5
+
+XOFF=$($INNER display-message -p -t "$FLOAT" '#{pane_left}')
+YOFF=$($INNER display-message -p -t "$FLOAT" '#{pane_top}')
+GRABCOL=$((XOFF + 3))
+BORDERROW=$YOFF
+
+# Plain (non-Alt) top-border drag: "MouseDrag1Border" -> resize-pane -M ->
+# a move, since grabbing the top border moves rather than resizes. Several
+# small steps, each its own drag-motion event and so its own pass through
+# the code under test.
+mouse 0 "$GRABCOL" "$BORDERROW" M
+i=0
+while [ $i -lt 6 ]; do
+	GRABCOL=$((GRABCOL + 1))
+	mouse 32 "$GRABCOL" "$BORDERROW" M
+	i=$((i + 1))
+done
+mouse 0 "$GRABCOL" "$BORDERROW" m
+sleep 0.3
+
+NEWXOFF=$($INNER display-message -p -t "$FLOAT" '#{pane_left}')
+[ "$NEWXOFF" != "$XOFF" ] || fail "sanity: floating pane did not move (still at $XOFF)"
+
+LOG=$(ls tmux-server*.log 2>/dev/null | head -1)
+[ -n "$LOG" ] || fail "sanity: no server -vv log was produced"
+
+n=$(grep -c "redraw deferred (8 left)" "$LOG")
+[ "$n" -eq 0 ] ||
+	fail "drag self-deferred against its own queued sync bytes $n time(s)"
+
+exit 0

server-client.c

@@ -2429,8 +2429,19 @@ server_client_check_redraw(struct client *c)
 	 * that clear and forces a full catch-up redraw once this client is
 	 * unblocked, rather than trying to keep the fine-grained state
 	 * around for a retry.
+	 *
+	 * If a synchronized-output frame is open, discount anything queued
+	 * since it started (down to sync_offset, the length when it opened):
+	 * those bytes are already part of the frame this pass is committed
+	 * to flushing (see tty_sync_start()), not a reason to defer this
+	 * pass's redraw - without this, a mouse-drag callback that itself
+	 * opens the frame before writing anything would see its own
+	 * just-queued bytes as "outstanding output" and defer against
+	 * itself every single motion event.
 	 */
 	n = EVBUFFER_LENGTH(tty->out);
+	if ((tty->flags & TTY_SYNCING) && n > tty->sync_offset)
+		n = tty->sync_offset;
 	if (n != 0 || (tty->flags & TTY_BLOCK)) {
 		if (n != 0)
 			log_debug("%s: redraw deferred (%zu left)", c->name, n);

tmux.h

@@ -1802,6 +1802,14 @@ struct tty {
 	struct event	 timer;
 	size_t		 discarded;
 
+	/*
+	 * Buffer length at the instant a synchronized-output frame opened
+	 * (tty_sync_start()), so server_client_check_redraw()'s "is there
+	 * already outstanding output" check can discount whatever this
+	 * pass itself queued into that frame - see tty_sync_start().
+	 */
+	size_t		 sync_offset;
+
 	struct termios	 tio;
 
 	struct grid_cell cell;

tty.c

@@ -1519,6 +1519,7 @@ tty_sync_start(struct tty *tty)
 	if (tty->flags & TTY_SYNCING)
 		return;
 	tty->flags |= TTY_SYNCING;
+	tty->sync_offset = EVBUFFER_LENGTH(tty->out);
 
 	if (tty_term_has(tty->term, TTYC_SYNC)) {
 		log_debug("%s sync start", tty->client->name);