Boostrap Buttons Wrapping when they shouldn't be

I can't see why the Search and Clear Buttons wrap onto the following line after Postcode.

I've used the last example from the Button Addons code from here.

https://getbootstrap.com/docs/4.0/components/input-group/.

Whilst I have not included aria-label="Recipient's username" aria-describedby="basic-addon2" as R htmltools doesn't seem to like it, I don't think it's that (as I've tried adding it in the console with no effect).

Although the Bootstrap example doesn't explicitly add the -fluid to container and flex-xl-nowrap to row, I can see their code has it in Console so tried adding it.

I've added background colours to the columns to make it clear where they are.

---
title: "Stack Overflow Bootstrap "
output: html_document
---

## Purpose
Show buttons wrapping when I don't want them to
```{r cars, echo=FALSE}

# Load packages
pacman::p_load(tidyverse, htmltools)

  tagList(
    htmltools::withTags(
      div(class="container-fluid",
        h2("Heading 2"),
        div(class="row flex-xl-nowrap",
          div(class="col-sm-6", 
              style="background-color:powderblue;",
            h3("1st Column Heading 3")),
          div(class="col-sm-6", 
              style="background-color:Cornsilk;",
            h3("2nd Column Heading 3"),
            div(class="input-group", 
              input(
                type = "text",
                class="form-control",
                placeholder = "Postcode...",
                style = "width: 80px;"
              ),
              div(class="input-group-append",
                button(
                  type = "submit",
                  class="btn btn-success",
                  "Search"
                ),
                button(
                  type = "reset",
                  class="btn btn-primary",
                  "Clear"
                )
              )
            )
          )
        )
      )
    )
  )
```

As @RadovanMiletić pointed out, the answer is to add "display: block", i.e. change

div(class="input-group",

to

div(class="input-group", style = "display: block",