A partial response to this tweet...

library(sf)
library(plotly)
library(leaflet)
library(crosstalk)
library(htmltools)

boroughs<- st_read("http://services5.arcgis.com/GfwWNkhOj9bNBqoJ/arcgis/rest/services/nybb/FeatureServer/0/query?where=1=1&outFields=*&outSR=4326&f=geojson")
boroughs$x <- seq(1:5)
boroughs$y <- seq(2,10,2)

boroughs_sd <- SharedData$new(
  boroughs,
  key=~BoroCode,
  # provide explicit group so we can easily refer to this later
  group = "boroughs"
)

map <- leaflet(boroughs_sd) %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addPolygons(
    data=boroughs,
    layerId = ~BoroCode,
    color = "#444444",
    weight = 1,
    smoothFactor = 0.5,
    opacity = 1.0,
    fillOpacity = 0.5,
    fillColor = ~colorQuantile("Greens", x)(x)#,
#  turn off highlight since it interferes with selection styling
#   if careful with styling could have both highlight and select
#    highlightOptions = highlightOptions(color = "white", weight = 2)
  )

# borrow from https://github.com/r-spatial/mapedit/blob/master/R/query.R#L73-L132
#   to select/deselect features but instead of Shiny.onInputChange
#   use crosstalk to manage state
add_select_script <- function(lf, styleFalse, styleTrue, ns="") {
  ## check for existing onRender jsHook?

  htmlwidgets::onRender(
    lf,
    sprintf(
"
function(el,x) {
  var lf = this;
  var style_obj = {
    'false': %s,
    'true': %s
  }

  // instead of shiny input as our state manager
  //   use crosstalk
  if(crosstalk) {
    var ct_sel = new crosstalk.SelectionHandle()
    ct_sel.setGroup('boroughs')
    ct_sel.on('change', function(x){
      if(x.sender !== ct_sel) { //ignore select from this map
        lf.eachLayer(function(lyr){
          if(lyr.options && lyr.options.layerId) {
            var id = String(lyr.options.layerId)
            if(
              !x.value  ||
              (Array.isArray(x.value) && x.value.indexOf(id) === -1)
            ) {
              toggle_state(lyr, false)
              toggle_style(lyr, style_obj.false)
            }
            if(Array.isArray(x.value) && x.value.indexOf(id) > -1) {
              toggle_state(lyr, true)
              toggle_style(lyr, style_obj.true)
            }
          }
        })
      }
    })
  }

  // define our functions for toggling
  function toggle_style(layer, style_obj) {
    layer.setStyle(style_obj);
  };
  function toggle_state(layer, selected, init) {
    if(typeof(selected) !== 'undefined') {
      layer._mapedit_selected = selected;
    } else {
      selected = !layer._mapedit_selected;
      layer._mapedit_selected = selected;
    }
    if(typeof(Shiny) !== 'undefined' && Shiny.onInputChange && !init) {
      Shiny.onInputChange(
        '%s-mapedit_selected',
        {
          'group': layer.options.group,
          'id': layer.options.layerId,
          'selected': selected
        }
      )
    }

    if(ct_sel) {
      var ct_values = ct_sel.value
      var id = String(layer.options.layerId)
      if(selected) {
        if(!ct_values) {
          ct_sel.set([id])
        }
        if(Array.isArray(ct_values) && ct_values.indexOf(id) === -1) {
          ct_sel.set(ct_values.concat(id))
        }
      }

      if(ct_values && !selected) {
        ct_values.length > 1 ?
          ct_sel.set(
            ct_values.filter(function(d) {
              return d !== id
            })
          ) :
          ct_sel.set(null) // select all if nothing selected
      }
    }

    return selected;
  };
  // set up click handler on each layer with a group name
  lf.eachLayer(function(lyr){
    if(lyr.on && lyr.options && lyr.options.layerId) {
      // start with all unselected ?
      toggle_state(lyr, false, init=true);
      toggle_style(lyr, style_obj[lyr._mapedit_selected]);
      lyr.on('click',function(e){
        var selected = toggle_state(e.target);
        toggle_style(e.target, style_obj[String(selected)]);
      });
    }
  });
}
",
      jsonlite::toJSON(styleFalse, auto_unbox=TRUE),
      jsonlite::toJSON(styleTrue, auto_unbox=TRUE),
      ns
    )
  )
}

browsable(
  tagList(
    tags$div(
      style = "float:left; width: 49%;",
      add_select_script(
        map,
        styleFalse = list(fillOpacity = 0.2, weight = 1, opacity = 0.4, color="black"),
        styleTrue = list(fillOpacity = 0.7, weight = 3, opacity = 0.7, color="blue")
      )
    ),
    tags$div(
      style = "float:left; width: 49%;",
      plot_ly(boroughs_sd, x = ~x, y = ~y) %>%
        add_markers(alpha = 0.5,text = ~paste('Borough: ', BoroName)) %>%
        highlight(on = "plotly_selected")
    )
  )
)