Unable to embed Bookdown site in R Shiny app

Question

I have a Bookdown site that I would like to embed in an R Shiny app, however I am having an issue where only the top toolbar is rendering correctly. I am using a basic Bookdown template and rendering the site using the gitbook option, then trying to embed the html files into the R Shiny app, as shown in my code below.

When displayed in fullscreen, only the top toolbar is visible:

When the window is made smaller, it becomes apparent that the body of the content is being rendered, but in an extremely small box up near the top:

It's unclear why this is happening and I've tried this with larger R Shiny apps as well as the minimalist example below. I initially tried this using a larger Bookdown site and then with smaller examples including the single-page site in the reprex here. What is causing this odd rendering and is it possible to fix this within a Shiny app environment?

server.R file

library(shiny)
library(htmltools)

shinyServer(function(input, output, session) {

  output$ui <- renderUI({
    htmltools::includeHTML("./www/index.html")
  })

})

ui.R file

library(shiny)

shinyUI(
  uiOutput("ui")
)

_bookdown.yml

book_filename: "Bookdown Reprex"
output_dir: www
delete_merged_file: true
language:
  label:
    fig: "FIGURE "
    tab: "TABLE "
  ui:
    chapter_name: ""

_output.yml

bookdown::pdf_book:
  keep_tex: yes
  latex_engine: xelatex
  pandoc_args: --top-level-division=chapter
  toc_depth: 2
  toc_unnumbered: no
bookdown::gitbook:
  css: css/style.css
  split_by: chapter
  lib_dir: book_assets
  config:
    toc:
      collapse: chapter
    download: pdf
    sharing: no

index.Rmd

---
title: "Bookdown Reprex"
date: "`r Sys.Date()`"
fontsize: 11pt
geometry: margin=1in
mainfont: FreeSans
site: bookdown::bookdown_site
always_allow_html: yes
colorlinks: yes
---

Introduction
============

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, 
PDF, and MS Word documents. For more details on using R Markdown see 
<http://rmarkdown.rstudio.com>.

When you click the **Knit** button a document will be generated that includes both content 
as well as the output of any embedded R code chunks within the document. You can embed an 
R code chunk like this:

```{r cars}
summary(cars)
```

style.css

p.caption{
  color: #777;
  margin-top: 10px;
}
p code {
  white-space: inherit;
}
pre {
  word-break: normal;
  word-wrap: normal;
}
pre code {
  white-space: inherit;
}

Answer 1

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