## ----eval = FALSE-------------------------------------------------------------
#  ui = fluidPage(
#      ...,
#      plotOutput("heatmap", click = "heatmap_click")
#  )
#  server = function(input, output, session) {
#      ht_obj = reactiveVal(NULL)
#      ht_pos_obj = reactiveVal(NULL)
#      output$heatmap = renderPlot({
#          ...
#          ht = draw(Heatmap(mat))
#          ht_pos = htPositionsOnDevice(ht)
#          ht_obj(ht)
#          ht_pos_obj(ht_pos)
#      })
#      observeEvent(input$heatmap_click, {
#          pos = getPositionFromClick(input$heatmap_click)
#          df = selectPosition(ht_obj(), pos, mark = FALSE,
#              ht_pos = ht_pos_obj(), verbose = FALSE)
#          # do something with `df`
#          ...
#      })
#  }