Browser-friendly way of drawing rectangles on top of image R Shiny

Question

I have written a shiny app that allows the user to draw rectangles on top of an image (minimal reproducible example below).

The problem with my current approach is t

Answer 1

Here's a JS option based entirely on this answer.

# JS and CSS modified from: https://stackoverflow.com/a/17409472/8099834
css <- "
    #canvas {
        width:2000px;
        height:2000px;
        border: 10px solid transparent;
    }
    .rectangle {
        border: 5px solid #FFFF00;
        position: absolute;
    }
"

js <- 
"function initDraw(canvas) {
    var mouse = {
        x: 0,
        y: 0,
        startX: 0,
        startY: 0
    };
    function setMousePosition(e) {
        var ev = e || window.event; //Moz || IE
        if (ev.pageX) { //Moz
            mouse.x = ev.pageX + window.pageXOffset;
            mouse.y = ev.pageY + window.pageYOffset;
        } else if (ev.clientX) { //IE
            mouse.x = ev.clientX + document.body.scrollLeft;
            mouse.y = ev.clientY + document.body.scrollTop;
        }
    };

    var element = null;    
    canvas.onmousemove = function (e) {
        setMousePosition(e);
        if (element !== null) {
            element.style.width = Math.abs(mouse.x - mouse.startX) + 'px';
            element.style.height = Math.abs(mouse.y - mouse.startY) + 'px';
            element.style.left = (mouse.x - mouse.startX < 0) ? mouse.x + 'px' : mouse.startX + 'px';
            element.style.top = (mouse.y - mouse.startY < 0) ? mouse.y + 'px' : mouse.startY + 'px';
        }
    }

    canvas.onclick = function (e) {
        if (element !== null) {
           var coord = {
               left: element.style.left,
               top: element.style.top,
               width: element.style.width,
               height: element.style.height
            };
            Shiny.onInputChange('rectCoord', coord);
            element = null;
            canvas.style.cursor = \"default\";
        } else {
            mouse.startX = mouse.x;
            mouse.startY = mouse.y;
            element = document.createElement('div');
            element.className = 'rectangle'
            element.style.left = mouse.x + 'px';
            element.style.top = mouse.y + 'px';
            canvas.appendChild(element);
            canvas.style.cursor = \"crosshair\";
        }
    }
};
$(document).on('shiny:sessioninitialized', function(event) {
    initDraw(document.getElementById('canvas'));
});
"

library(shiny)

ui <- fluidPage(
  tags$head(
      tags$style(css),
      tags$script(HTML(js))
  ),
  fluidRow(
      column(width = 6, 
             # inline is necessary
             # ...otherwise we can draw rectangles over entire fluidRow
             uiOutput("canvas", inline = TRUE)),
      column(
          width = 6,
          verbatimTextOutput("rectCoordOutput")
          )
  )
)

server <- function(input, output, session) {
    output$canvas <- renderUI({
        tags$img(src = "https://www.r-project.org/logo/Rlogo.png")
    })
    output$rectCoordOutput <- renderPrint({
        input$rectCoord
    })

}

shinyApp(ui, server)