Extract inputBorderFacesList helper

Facing-side rule was written out separately in options.go and
terminal.go; single helper prevents the two from drifting apart

src/terminal.go

@@ -1713,10 +1713,21 @@ func (t *Terminal) separatorLength() int {
 
 // The default horizontal separator is redundant if the input section is
 // already visually separated from the list section by a border line
+// Whether the input border draws a line on the side facing the list section.
+// BorderLine is placed on the facing side when resolved in NewTerminal.
+func inputBorderFacesList(layout layoutType, shape tui.BorderShape) bool {
+	if shape == tui.BorderLine {
+		return true
+	}
+	if layout == layoutReverse {
+		return shape.HasBottom()
+	}
+	return shape.HasTop()
+}
+
 func (t *Terminal) separatedByBorder() bool {
 	// The input border itself draws a line on the side facing the list section
-	if t.layout == layoutReverse && t.inputBorderShape.HasBottom() ||
-		t.layout != layoutReverse && t.inputBorderShape.HasTop() {
+	if inputBorderFacesList(t.layout, t.inputBorderShape) {
 		return true
 	}
 	for po := &t.previewOpts; po != nil; po = po.alternative {