diff --git a/tests/Terminal_-_Detect_width.sh b/tests/Terminal_-_Detect_width.sh
new file mode 100644
index 0000000..63389cb
--- /dev/null
+++ b/tests/Terminal_-_Detect_width.sh
@@ -0,0 +1,67 @@
+#!/bin/sh
+#
+# Check that the terminal width is detected on startup.
+
+# Dummy assignments for "shellcheck".
+testSubject="${testSubject:-false}"; workFile1="${workFile1:-.tmp1}"; workFile2="${workFile2:-.tmp2}"
+
+# Skip the test if `tmux' is not available.
+if ! command -v tmux >/dev/null 2>&1; then
+	echo "test requires \`tmux'"
+	exit 2
+fi
+
+# Skip the test if `tmux' does not have "-C".
+if echo "kill-server" | tmux -C -L pvtest 2>&1 | grep -Fq "tmux: unknown option"; then
+	echo "test requires a newer \`tmux'"
+	exit 2
+fi
+
+# Run a progress meter inside a terminal of the given width, and output the
+# longest number of characters seen between "[" and "]".
+#
+runWidthTest () {
+	terminalWidth="$1"
+
+	{
+	echo "set remain-on-exit on"
+	echo "new-session -d"
+	echo "resize-window -x ${terminalWidth} -y 5"
+	echo "pipe-pane 'cat > ${workFile1}'"
+	echo "respawn-pane -k sh -c \"${testSubject} -pSs 1K </dev/zero >/dev/null\""
+	sleep 1
+	echo "kill-server"
+	sleep 1
+	} \
+	| tmux -C -L pvtest >/dev/null
+
+	tr '\r' '\n' < "${workFile1}" \
+	| sed -n 's/^.*\[//;s/\].*$//p' \
+	| awk 'BEGIN {m=0} {n=length($0); m=(n>m?n:m)} END {print m}'
+}
+
+
+# Perform a test run of the given width and exit on failure.  The longest
+# bar found by runWidthTest() should be the width of the terminal minus the
+# "[] 100%" characters, i.e. 7 less.
+#
+widthTest () {
+	terminalWidth="$1"
+	longestBar=$(runWidthTest "${terminalWidth}")
+	if test -z "${longestBar}"; then
+		echo "no output from test at width=${terminalWidth}"
+		exit 1
+	elif ! test "${longestBar}" -eq $((terminalWidth-7)); then
+		echo "bar size incorrect (${longestBar}) at width=${terminalWidth})"
+		exit 1
+	fi
+}
+
+widthTest 80
+widthTest 120
+widthTest 400
+widthTest 20
+
+exit 0
+
+# EOF