Fix a bug in the Go code of the CSI key event parser

Fixes #6189

tools/tui/loop/key-encoding_test.go

@@ -0,0 +1,30 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package loop
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+var _ = fmt.Print
+
+func TestKeyEventFromCSI(t *testing.T) {
+
+	test_text := func(csi string, expected, alternate string) {
+		ev := KeyEventFromCSI(csi)
+		if ev == nil {
+			t.Fatalf("Failed to get parse %#v", csi)
+		}
+		if diff := cmp.Diff(expected, ev.Text); diff != "" {
+			t.Fatalf("Failed to get text from %#v:\n%s", csi, diff)
+		}
+		if diff := cmp.Diff(alternate, ev.AlternateKey); diff != "" {
+			t.Fatalf("Failed to get alternate from %#v:\n%s", csi, diff)
+		}
+	}
+	test_text("121;;121u", "y", "")
+	test_text("121::122;;121u", "y", "z")
+}