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

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()
```

Start with the supplied flexdashboard document using shiny::fillCol() and DT::datatable() with the Scroller extension, then compare rendering between flexdashboard 0.5.2 and 0.6.1. Trace the rendering path for the fillCol entry point and verify the table no longer shows incorrect sizing or the extra "Showing X to Y of Z entries" output.