Expand/Collapse Shiny selectInput function

Question

I would like to find a resource that would allow my Shiny selectInput function to expand/collapse based on the category headings that I have created. I have searched through some bootstrap resources, but am not yet successful. Please forgive my minimal working example, I acknowledge that there may be more efficient ways to provide a MWE. Thanks for any advice you can offer.

library(shiny)
library(tidyverse)
#create a quick dataset to plot
schools <-  as.data.frame(table(
    c('Adams', 'Van Buren', 'Clinton', 'Douglas', 'Edwards', 
              'Franklin', 'Grant', 'Harrison', 'Ignatius', 'Justice', 
              'Kellogg', 'Lincoln'), 
    dnn = list("school")))

enrollment <- as.data.frame(table(
    c(300, 305, 265, 400, 500, 450, 475, 900, 800, 850, 1200, 1500), 
    dnn = list("enrollment")))

schoolsDataframe <- schools %>% 
    bind_cols(enrollment) %>% 
    select(school, enrollment)

#define data elements for selectInput choices argument
elem <- c('Adams', 'Van Buren', 'Clinton', 'Douglas')
mid <- c('Edwards', 'Franklin', 'Grant')
high <- c('Harrison', 'Ignatius', 'Justice')
multi <- c('Kellogg', 'Lincoln')

# Define UI 
ui <- fluidPage(
    tags$style(".optgroup-header { color: #FFFFFF !important; background: #000000 !important; }"),
    # Application title
    titlePanel("Expandable selectInput"),

    # Sidebar with a select input
    sidebarLayout(
        sidebarPanel(
            selectInput(inputId = 'schoolsInput',
                        label = 'Select a school',
                        choices = list('Elementary' = elem, 
                                       'Middle' = mid, 
                                       'High' = high, 
                                       'Multi-level' = multi), 
                        selectize = TRUE)
        ),

        # Show a plot 
        mainPanel(
           plotOutput("myPlot")
        )
    )
)

# Define server logic required to draw a plot
server <- function(input, output) {

    output$myPlot <- renderPlot({
        #filter the data based on selectInput
schoolsDataframe <- schoolsDataframe %>% 
    filter(school == input$schoolsInput)
        # draw the plot
ggplot(data = schoolsDataframe, 
       mapping = aes(x = school, 
                     y = enrollment))+
    geom_col()
    })
}

# Run the application 
shinyApp(ui = ui, server = server)

Answer 1

library(shiny)

onInitialize <- '
function(){
  this.$dropdown_content.on("mousedown", function(e){
    e.preventDefault(); 
    return false;
  }); 
  $("body").on("click", ".optgroup-header", function(){
    $(this).siblings().toggle();
  });
}'

onDropdownOpen <- '
function(){
  setTimeout(function(){
    $(".optgroup .option").hide();
  }, 0);
}'

shinyApp(

  ui = fluidPage(
    selectizeInput("state", "Choose a state:",
                list(`East Coast` = list("NY", "NJ", "CT"),
                     `West Coast` = list("WA", "OR", "CA"),
                     `Midwest` = list("MN", "WI", "IA")),
                options = list(
                  onInitialize = I(onInitialize),
                  onDropdownOpen = I(onDropdownOpen)
                )
    ),
    textOutput("result")
  ),

  server = function(input, output) {
    output$result <- renderText({
      paste("You chose", input$state)
    })
  }

)

Answer 2

The answer from Stéphane Laurent is fantastic, but it only works when there is one single dropdown on the page. If you have more than one dropdown, here is a slightly modified version of his answer that works with multiple inputs:

library(shiny)

onInitialize <- '
$(function() {
  $("body").on("mousedown", ".selectize-dropdown-content", function(e){
    e.preventDefault(); 
    return false;
  }); 
  $("body").on("click", ".optgroup-header", function(){
    $(this).siblings().toggle();
  });
});'

onDropdownOpen <- '
function(el){
  setTimeout(function(){
    $(el).find(".optgroup .option").hide();
  }, 0);
}'

shinyApp(
  
  ui = fluidPage(
    tags$script(HTML(onInitialize)),
    selectizeInput("state", "Choose a state:",
                   list(`East Coast` = list("NY", "NJ", "CT"),
                        `West Coast` = list("WA", "OR", "CA"),
                        `Midwest` = list("MN", "WI", "IA")),
                   options = list(
                     onDropdownOpen = I(onDropdownOpen)
                   )
    ),
    textOutput("result"),
    selectizeInput("state2", "Choose a state:",
                   list(`East Coast` = list("NY", "NJ", "CT"),
                        `West Coast` = list("WA", "OR", "CA"),
                        `Midwest` = list("MN", "WI", "IA")),
                   options = list(
                     onDropdownOpen = I(onDropdownOpen)
                   )
    ),
    textOutput("result2")
  ),
  
  server = function(input, output) {
    output$result <- renderText({
      paste("You chose", input$state)
    })
    output$result2 <- renderText({
      paste("You chose", input$state2)
    })
  }
  
)