Use escape sequence in prepareArea

Add SaveCursorPosition and
RestoreCurcorPosition to output interafce
and use it in prepareArea
Fix #70

Co-authored-by: Masashi Shibata <c-bata@users.noreply.github.com>

output.go

@@ -146,6 +146,10 @@ type ConsoleWriter interface {
 	ScrollDown()
 	// ScrollUp scroll display up one line.
 	ScrollUp()
+	// SaveCursorPosition saves current cursor position.
+	SaveCursorPosition()
+	// RestoreCursorPosition restores cursor position.
+	RestoreCursorPosition()
 
 	/* Title */
 

output_vt100.go

@@ -174,6 +174,16 @@ func (w *VT100Writer) ScrollUp() {
 	w.WriteRaw([]byte{0x1b, 'M'})
 }
 
+// SaveCursorPosition saves current cursor position.
+func (w *VT100Writer) SaveCursorPosition() {
+	w.WriteRaw([]byte{0x1b, '[', 's'})
+}
+
+// RestoreCursorPosition restores cursor position.
+func (w *VT100Writer) RestoreCursorPosition() {
+	w.WriteRaw([]byte{0x1b, '[', 'u'})
+}
+
 /* Title */
 
 // SetTitle sets a title of terminal window.

render.go

@@ -69,12 +69,12 @@ func (r *Render) TearDown() {
 }
 
 func (r *Render) prepareArea(lines int) {
+	r.out.SaveCursorPosition()
+
 	for i := 0; i < lines; i++ {
-		r.out.ScrollDown()
-	}
-	for i := 0; i < lines; i++ {
-		r.out.ScrollUp()
+		r.out.WriteStr("\n")
 	}
+	r.out.RestoreCursorPosition()
 }
 
 // UpdateWinSize called when window size is changed.