R shiny app with mousewheel scrollable slider.

server.R

library(shiny)

server <- function(input, output, session) {
  
  observe({
    n <- input$n
    if(is.null(input$n)) {
      return()
    }
    change <- sign(input$dy)
    isolate(updateSliderInput(session, "slider", value = input$slider + change))
  })
  
}

ui.R

library(shiny)

ui <- fluidPage(
  includeScript("wheel.js"),
  div(class = "shinyScroll",
    sliderInput("slider", "Hover and move me with the Mouse Wheel!", min = 0, max = 10, value = 2)
  ),
  div(
    "Hovering and scrolling here will do nothing"
  ),
  div(class = "shinyScroll",
    "Hovering and scrolling here will also move the slider"
  )
)

wheel.js

$(document).ready(function() {
  
  var scrollable = document.getElementsByClassName("shinyScroll");
  
  for(var i = 0; i < scrollable.length; i++) {
    scrollable[i].addEventListener("wheel", shinyScroll);
  }
  
  var n = 0;
  
  function shinyScroll(event) {
    n++;
    Shiny.onInputChange("n", n);
    Shiny.onInputChange("dy", event.deltaY);
  }

});