Storyboard menu disappears on click

[cpsievert]

Introduced by #308 (i.e., regression from currenlt CRAN version). Interestingly appears to only be an issue for runtime: shiny

---
title: "Flexdashboard theming demo"
runtime: shiny
output: 
  flexdashboard::flex_dashboard:
    orientation: rows
---

Storyboard {.storyboard}
=========================================

### Plot 1

```{r}
library(ggplot2)
renderPlot(qplot(1:10))
```

*** 

Click on plot 2 story

### Plot 2

```{r}
renderPlot(qplot(1:10))
```

*** 

Storyboard disappears!
```