How to display a spinner before a JavaScript file is loaded?

I have a Shiny app which load a huge JavaScript file (>17Mo). I'd like show a spinner until the JavaScript file is loaded. That does not work with shinybusy: the spinner only appears after the JS file has been loaded.

library(shiny)
library(shinybusy)
library(bslib)

ui <- page_sidebar(
  tags$head(
    tags$script(src = "HUGE_JAVASCRIPT_FILE.js")
  ),
  
  busy_start_up(
    loader = spin_epic("orbit", color = "#FFF"),
    text = "Loading...",
    timeout = 1500,
    color = "#FFF",
    background = "#112446"
  ),
  
  title = "Gröbner implicitization with GIAC",
  
  sidebar = sidebar(
    textInput(
      "equations",
      "Equations: ",
      value = "x = a*cost, y = b*sint"
    ),
    textInput(
      "relations",
      "Relations: ",
      value = "cost^2 + sint^2 = 1"
    ),
    textInput(
      "symbols",
      "Symbols: ",
      value = "cost, sint, a, b"
    ),
    actionButton("go", "Run", class = "btn-primary")
  )
)

server <- function(input, output, session) { }

shinyApp(ui, server)

I would add the defer property and you can use the onload property to run some code once it is loaded, for instance Shiny.setInputValue to notify the server to close the spinner. We just have to introduce a small delay (setTimeout) , to make sure the Shiny framework is loaded (if somebody finds a solution which does not require this hack, I would be very happy to learn).

Setup

library(here)
library(data.table)

dir.create("www")
fn <- here("www", "larger.js")
fwrite(list(c(rep("console.log(1);", 2.5e4), "alert('done');")), fn)

Folder Structure

.
├── app.R
└── www
    └── larger.js

Code

library(shiny)
library(shinybusy)
library(bslib)

ui <- page_sidebar(
   tags$head(
      tags$script(HTML("function done() {
                         setTimeout(() => Shiny.setInputValue('done', true), 100)
                       }")),
      tags$script(src = "larger.js", defer = NA, onload = "done()")
   ),
   busy_start_up(
      loader = spin_epic("orbit", color = "#FFF"),
      text = "Loading...",
      mode = "manual",
      color = "#FFF",
      background = "#112446"
   ),
   
   title = "Gröbner implicitization with GIAC",
   
   sidebar = sidebar(
      textInput(
         "equations",
         "Equations: ",
         value = "x = a*cost, y = b*sint"
      ),
      textInput(
         "relations",
         "Relations: ",
         value = "cost^2 + sint^2 = 1"
      ),
      textInput(
         "symbols",
         "Symbols: ",
         value = "cost, sint, a, b"
      ),
      actionButton("go", "Run", class = "btn-primary")
   )
)

server <- function(input, output, session) {
   observeEvent(input$done, 
                remove_start_up())
}

shinyApp(ui, server)