tty: drop WT_SESSION-based Windows Terminal detection

nicm is opposed to identifying a terminal via an environment variable, so
remove the WindowsTerminal tty_default_features() entry and the
WT_SESSION check in tty_term_create() entirely, pending some other
accepted identification mechanism for Windows Terminal (it cannot be
identified via the existing XTVERSION path at all - see the previous
commit's now-removed comment for why).

This leaves WezTerm and ghostty's margins grants in place - both are
identified via the existing, already-accepted XTVERSION mechanism, with
DECSLRM support confirmed directly in their own source, independent of
this change.

regress/tty-margins-wt-session.sh tested only the removed mechanism and
is replaced by regress/tty-margins-scrollbar.sh, which checks the same
underlying scrollbar/margins scroll-decision mechanism (still real, and
still what WezTerm's and ghostty's table entries rely on) via the
generic terminal-features option instead of simulating any one
terminal's identification handshake.

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

regress/tty-margins-scrollbar.sh

@@ -0,0 +1,117 @@
+#!/bin/sh
+
+# A pane that doesn't span the terminal's full width - because
+# pane-scrollbars is on (the scrollbar occupies a column) or the pane is
+# one of a side-by-side split - needs DECSLRM (left/right margin) support
+# to use the fast native-scroll path (tty_cmd_linefeed()/scrollup()/
+# scrolldown()/reverseindex(), tty.c:
+# "(!tty_full_width(tty, ctx) && !tty_use_margin(tty))"). Without it, every
+# single scroll falls back to tty_redraw_region()'s full manual repaint of
+# the whole region - a real, confirmed source of flicker (and, separately,
+# of image content not surviving a scroll in branches with image support).
+#
+# tty_default_features() (tty-features.c) grants the "margins" feature to
+# several terminals it can positively identify via XTVERSION/DA2 (mintty,
+# iTerm2, WezTerm, ghostty, XTerm-as-VT420) - this checks the actual
+# server-side scroll decision via the -vv log for the underlying mechanism
+# those table entries all rely on, using the terminal-features option
+# directly (which any of them - or a user's own terminal-overrides -
+# ultimately feed into) rather than simulating any one terminal's
+# identification handshake.
+
+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 -Lmarginsscrollbar-inner-$$ -f/dev/null"
+OUTER="$TEST_TMUX -Lmarginsscrollbar-outer-$$ -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
+
+wait_for_client()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		CLIENT=$($INNER list-clients -F '#{client_name}' 2>/dev/null)
+		[ -n "$CLIENT" ] && return 0
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "inner client did not attach"
+}
+
+run_scroll_phase()
+{
+	label=$1
+	margins=$2
+
+	rm -f tmux-server*.log
+
+	$INNER new-session -d -s inner -x 40 -y 6 'exec sh' || exit 1
+	$INNER set -g status off || exit 1
+	$INNER set -g window-size manual || exit 1
+	$INNER set -g pane-scrollbars on || exit 1
+	if [ "$margins" = "on" ]; then
+		$INNER set -as terminal-features ',*:margins' || exit 1
+	fi
+
+	$OUTER new-session -d -x 40 -y 6 || exit 1
+	OUTERPANE=$($OUTER list-panes -F '#{pane_id}') || exit 1
+	$OUTER set -g status off || exit 1
+	$OUTER set -g window-size manual || exit 1
+	$OUTER set -g default-terminal screen-256color || exit 1
+	$OUTER send-keys -t "$OUTERPANE" -l "$INNER attach -t inner" || exit 1
+	$OUTER send-keys -t "$OUTERPANE" Enter || exit 1
+	sleep 1
+
+	wait_for_client
+
+	i=0
+	while [ "$i" -lt 8 ]; do
+		$INNER send-keys -t inner Enter || exit 1
+		sleep 0.2
+		i=$((i + 1))
+	done
+	sleep 0.3
+
+	LOG=$(ls tmux-server*.log 2>/dev/null | head -1)
+	[ -n "$LOG" ] || fail "$label: sanity: no server -vv log was produced"
+
+	n=$(grep -c "tty_redraw_region.*large region redraw" "$LOG")
+
+	$OUTER kill-server 2>/dev/null
+	$INNER kill-server 2>/dev/null
+
+	echo "$n"
+}
+
+# Phase 1: margins granted - must never fall back to a full region redraw.
+n_with=$(run_scroll_phase "with margins" "on")
+[ "$n_with" -eq 0 ] ||
+	fail "with margins granted, scrolling a scrollbar-enabled pane still fell back to a full region redraw ($n_with times)"
+
+# Phase 2: sanity check - without margins, the same scenario must actually
+# hit the fallback, proving phase 1 wasn't accidentally trivial.
+n_without=$(run_scroll_phase "without margins" "off")
+[ "$n_without" -gt 0 ] ||
+	fail "sanity: without margins, scrolling a scrollbar-enabled pane never fell back to a full region redraw - this scenario no longer exercises the bug this test checks for"
+
+exit 0

tty-features.c

@@ -637,6 +637,7 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "extkeys,"
 			      "focus,"
 		  	      "hyperlinks,"
+			      "margins,"
 			      "usstyle"
 		},
 		{ .name = "ghostty",
@@ -645,6 +646,7 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "cstyle,"
 			      "extkeys,"
 			      "focus,"
+			      "margins,"
 			      "overline,"
 			      "hyperlinks,"
 			      "osc7,"
@@ -675,7 +677,7 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "cstyle,"
 			      "extkeys,"
 			      "focus"
-		}
+		},
 	};
 	u_int	i;