incorrect DT::datatable() rendering with "Scroller" extension

[nplatonov]

In flexdashboard-0.5.2 shiny::fillCol(flex=NA,...) rendered correct DT::datatable(..., extensions="Scroller",...). But since flexdashboard-0.6.0 this rendering is failed.

flexdasboard version 0.5.2:

flexdasboard version 0.6.1:

---
pagetitle: Scroller extension
runtime: shiny
output:
   flexdashboard::flex_dashboard:
      orientation: rows
---

```{r echo=TRUE}
'dummyTable' <- function() {
   DT::datatable(mtcars[,1:6],rownames=TRUE,extensions="Scroller"
                ,options=list(dom="ift",scroller=T
                             ,scrollY="calc(100vh - 400px)"))
}
```

# Incorrect table rendering and "Showing X to Y of Z entries" output. `flexdashboard` version is `r packageVersion("flexdashboard")`. `DT` version is `r packageVersion("DT")`.

## 

###

```{r echo=exists("fillCol"), eval=exists("fillCol")}
shiny::fillCol(flex=NA,DT::renderDT(dummyTable()))
```

###

```{r echo=TRUE}
dummyTable()
```