页脚未显示在R flexdashboard中.sidebar列的底部

Question

我想在R中的flexdashbard侧边栏底部的固定位置添加页脚。我正在尝试使用以下功能：

添加了CSS

---
title: "Some title"
runtime: shiny
output: 
  flexdashboard::flex_dashboard:
    orientation: columns
    vertical_layout: fill
---

<style>
.navbar {
  border-color:black;
}
.navbar-brand {
color:black!important;
}
.footer {
  grid-area: footer;
  position: sticky;
  position: -webkit-sticky;
  position: fixed;
  bottom: 0;
  z-index: 1000;
}
</style>



```{r setup, include=FALSE}
library(flexdashboard)
```

DATA
=====================================

Column {.sidebar}
-----------------------------------------------------------------------

```{r}
# I tried this
tags$footer(title="Your footer here")

# I also tried this
HTML('<footer> Your text
 </footer>')

# I also tried this
tags$div( HTML("<footer><small>&copy; 2018, My Co., USA</small></footer>"))
```

Column {data-width=650}
-----------------------------------------------------------------------

### Chart 2

```{r}
```   

### Chart 3

```{r}
```

在上面，我提到了添加CSS样式代码后尝试的所有选项。这些都不能让我返回我想要的东西。

为什么版权信息（页脚）没有出现在仪表板的左侧栏（列）的底部？