Fix half page down bug

src/view.go

@@ -179,9 +179,11 @@ func (v *View) HalfPageDown() {
 	if len(v.buf.lines)-(v.topline+v.height) > v.height/2 {
 		v.ScrollDown(v.height / 2)
 	} else {
+		if len(v.buf.lines) >= v.height {
 			v.topline = len(v.buf.lines) - v.height
 		}
 	}
+}
 
 // CanClose returns whether or not the view can be closed
 // If there are unsaved changes, the user will be asked if the view can be closed