Add copy-mode -c to initialize the preferred cursor column

CHANGES

@@ -1,5 +1,8 @@
 CHANGES FROM 3.7c TO 3.8
 
+* Add copy-mode -c to start with the current cursor column as the preferred
+  column, rather than initially following line ends.
+
 * Many improvements to floating panes:
 
   - new-pane and split-window can now set the title with -T and border lines

cmd-copy-mode.c

@@ -30,8 +30,8 @@ const struct cmd_entry cmd_copy_mode_entry = {
 	.name = "copy-mode",
 	.alias = NULL,
 
-	.args = { "dekHMqSs:t:u", 0, 0, NULL },
+	.args = { "cdekHMqSs:t:u", 0, 0, NULL },
-	.usage = "[-dekHMqSu] [-s src-pane] " CMD_TARGET_PANE_USAGE,
+	.usage = "[-cdekHMqSu] [-s src-pane] " CMD_TARGET_PANE_USAGE,
 
 	.source =  { 's', CMD_FIND_PANE, 0 },
 	.target = { 't', CMD_FIND_PANE, 0 },

regress/copy-mode-entry-column.sh

@@ -0,0 +1,190 @@
+#!/bin/sh
+
+PATH=/bin:/usr/bin
+TERM=screen
+LC_ALL=C.UTF-8
+export PATH TERM LC_ALL
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Lentry-column-$$ -f/dev/null"
+
+fail()
+{
+	echo "$mode: $*" >&2
+	exit 1
+}
+
+cleanup()
+{
+	$TMUX kill-server 2>/dev/null
+}
+trap cleanup 0 1 15
+
+x()
+{
+	$TMUX send-keys -X "$@" || fail "copy command failed: $*"
+}
+
+check_cursor()
+{
+	actual=$($TMUX display-message -p '#{copy_cursor_x},#{copy_cursor_y}')
+	[ "$actual" = "$1" ] || fail "expected cursor $1, got $actual"
+}
+
+wait_cursor()
+{
+	i=0
+	while [ "$i" -lt 50 ]; do
+		actual=$($TMUX display-message -p '#{cursor_x},#{cursor_y}')
+		[ "$actual" = "$1" ] && return 0
+		sleep 0.1
+		i=$((i + 1))
+	done
+	fail "pane cursor did not reach $1"
+}
+
+mode=emacs
+$TMUX new-session -d -x40 -y10 \
+    "printf '%s\r\n' ABCDEFGHIJKLMNOPQRST abc ABCDEFGHIJKLMNOPQRST \
+	abcdefgh ABCDEFGHIJKLMNOP ab 01234567890123456789; \
+	printf '\033[4;9H'; exec cat" ||
+    fail "new-session failed"
+$TMUX set-option -g status off || fail "set status failed"
+$TMUX set-option -g window-size manual || fail "set window-size failed"
+wait_cursor 8,3
+
+for mode in emacs vi; do
+	$TMUX set-window-option -g mode-keys "$mode" ||
+	    fail "set mode-keys failed"
+
+	# Ordinary entry still follows line ends; -c starts at the same physical
+	# position but treats its column as a numeric preference.
+	for direction in up down; do
+		if [ "$direction" = up ]; then
+			row=2
+			end=20
+		else
+			row=4
+			end=16
+		fi
+		[ "$mode" = emacs ] || end=$((end - 1))
+		$TMUX copy-mode || fail "copy-mode failed"
+		check_cursor 8,3
+		x "cursor-$direction"
+		check_cursor "$end,$row"
+		x cancel
+
+		$TMUX copy-mode -c || fail "copy-mode -c failed"
+		check_cursor 8,3
+		x "cursor-$direction"
+		check_cursor "8,$row"
+		x cancel
+	done
+
+	# Short lines retain the existing emacs and vi movement rules. Emacs
+	# restores the preferred column; vi resumes from its last-character clamp.
+	$TMUX copy-mode -c || fail "copy-mode -c failed"
+	x cursor-up
+	check_cursor 8,2
+	x cursor-up
+	if [ "$mode" = emacs ]; then
+		check_cursor 3,1
+		restored=8
+	else
+		check_cursor 2,1
+		restored=2
+	fi
+	x cursor-up
+	check_cursor "$restored,0"
+	x cancel
+
+	$TMUX copy-mode -c || fail "copy-mode -c failed"
+	x cursor-down
+	check_cursor 8,4
+	x cursor-down
+	if [ "$mode" = emacs ]; then
+		check_cursor 2,5
+		restored=8
+	else
+		check_cursor 1,5
+		restored=1
+	fi
+	x cursor-down
+	check_cursor "$restored,6"
+	x cancel
+
+	# Horizontal movement before the first vertical move replaces the initial
+	# preference, rather than leaving the entry column permanently fixed.
+	$TMUX copy-mode -c || fail "copy-mode -c failed"
+	x cursor-left
+	check_cursor 7,3
+	x cursor-up
+	check_cursor 7,2
+	x cancel
+done
+
+# Passing -c while already in copy mode must not reinitialize its preference.
+mode=emacs
+$TMUX set-window-option -g mode-keys "$mode" || fail "set mode-keys failed"
+$TMUX copy-mode -c || fail "copy-mode -c failed"
+x cursor-left
+x cursor-up
+x cursor-up
+check_cursor 3,1
+$TMUX copy-mode -c || fail "second copy-mode -c failed"
+check_cursor 3,1
+x cursor-up
+check_cursor 7,0
+x cancel
+
+# An empty entry line has a numeric column zero with -c.
+$TMUX respawn-pane -k \
+    "printf '\033[H\033[2JABCDEFGHIJKLMNOPQRST\033[2;1H'; exec cat" ||
+    fail "respawn-pane failed"
+wait_cursor 0,1
+for mode in emacs vi; do
+	$TMUX set-window-option -g mode-keys "$mode" ||
+	    fail "set mode-keys failed"
+	$TMUX copy-mode -c || fail "copy-mode -c failed"
+	check_cursor 0,1
+	x cursor-up
+	check_cursor 0,0
+	x cancel
+done
+
+# The initial preference counts terminal columns, not characters.
+$TMUX respawn-pane -k \
+    "printf '\033[H\033[2J'; \
+	printf '%s\r\n' ABCDEFGHIJKLMNOPQRST ABCDEFGHIJKLMNOPQRST; \
+	printf 'abc中def'; exec cat" ||
+    fail "respawn-pane failed"
+wait_cursor 8,2
+for mode in emacs vi; do
+	$TMUX set-window-option -g mode-keys "$mode" ||
+	    fail "set mode-keys failed"
+	$TMUX copy-mode -c || fail "copy-mode -c failed"
+	check_cursor 8,2
+	x cursor-up
+	check_cursor 8,1
+	x cancel
+done
+
+# Initialize before the optional entry-time page movement.
+$TMUX respawn-pane -k \
+    "printf '\033[H\033[2JABCDEFGHIJKLMNOPQRST'; \
+	printf '\033[9;1Habcdefgh\033[10;1HABCDEFGHIJKLMNOP\033[9;9H'; \
+	exec cat" ||
+    fail "respawn-pane failed"
+wait_cursor 8,8
+for mode in emacs vi; do
+	$TMUX set-window-option -g mode-keys "$mode" ||
+	    fail "set mode-keys failed"
+	$TMUX copy-mode -cu || fail "copy-mode -cu failed"
+	check_cursor 8,0
+	x cancel
+	$TMUX copy-mode -cd || fail "copy-mode -cd failed"
+	check_cursor 8,9
+	x cancel
+done
+
+exit 0

tmux.1

@@ -2609,12 +2609,17 @@ The synopsis for the
 command is:
 .Bl -tag -width Ds
 .It Xo Ic copy\-mode
-.Op Fl dekHMqSu
+.Op Fl cdekHMqSu
 .Op Fl s Ar src\-pane
 .Op Fl t Ar target\-pane
 .Xc
 Enter copy mode.
 .Pp
+.Fl c
+initializes the preferred cursor column to the column at which copy mode is
+entered, rather than initially following the end of each line.
+Subsequent cursor movements use the normal copy mode behaviour.
+.Pp
 .Fl u
 enters copy mode and scrolls one page up and
 .Fl d

window-copy.c

@@ -614,6 +614,11 @@ window_copy_init(struct window_mode_entry *wme,
 	window_copy_sync_snapshot(data, base->grid);
 
 	data->cx = cx;
+	if (args_has(args, 'c')) {
+		/* Start with a preferred column rather than end of line. */
+		data->lastcx = cx;
+		data->lastsx = cx + 1;
+	}
 	if (cy < screen_hsize(data->backing)) {
 		data->cy = 0;
 		data->oy = screen_hsize(data->backing) - cy;