Collapsible checkboxGroupInput in shiny

I have created a set of checkbox using renderUI and checkboxGroupInput. Here the result:

What I would like to obtain now is something like this:

Where only the top results are shown with the possibility to expand the checkbox list.

Any suggestion on how to obtain this?

The code for the checkbox is the following:

Server.R:

    my_checkboxGroupInput <- function(variable, label,choices, selected, colors,perc){
    my_names <- choices
    log_funct("my_names",my_names,   verbose=T)
    if(length(names(choices))>0) my_names <- names(choices)
    log_funct("names(my_names)",my_names,   verbose=T)
    log_funct("choices",choices,   verbose=T)
     log_funct("selected",selected,   verbose=T)
    div(id=variable,class="form-group shiny-input-checkboxgroup shiny-input-container shiny-bound-input",
        HTML(paste0('<label class="control-label" for="',variable,'">',label,'</label>')),
        div( class="shiny-options-group",
             HTML(paste0('<div class="checkbox">',
                         '<label style="width: 100%">',
                         '<input type="checkbox" name="', variable, 
                         '" value="', choices, 
                         '"', ifelse(choices %in% selected, 'checked="checked"', ''), 
                         '/>',
                         #'<span ', ifelse(choices %in% selected, paste0('style=" background-color:',colors ,'; display: inline-block; white-space: nowrap; width: ',perc, '%;"'),''), '>',my_names,'</span>',
                         '<span ', paste0('style=" background-color:',colors ,'; display: inline-block; white-space: nowrap; width: ',perc, '%;"'),'>',my_names,'</span>',
                         '</label>',
                         '</div>', collapse = " "))
        )
    )
  }

  output$checkbox_cond <- renderUI({

    my_checkboxGroupInput("variable", "Variable:",choices = cond_plot()$Var1, 
                          selected=c(as.character(cond_plot()$Var1)[1],as.character(cond_plot()$Var1)[2]),
                          colors=c('#4e71ba'),
                          perc= cond_plot()$perc)
  })

The code is a modified version of the one in: how to make the checkboxgroupinput color-coded in Shiny