fix: Schedule .modal("hide") for transitioning modals

[gadenbuie]

Fixes posit-dev/py-shiny#1318 for real.

The core issue is that calling $el.modal('hide') on a modal element $el that hasn't been fully revealed yet doesn't work. In practice, this means that if a modal is shown and hidden within 300ms (the default modal transition time) the .modal('hide') call silently does nothing.

This solution in this PR is to use the show.bs.modal event that fires at the start of the modal transition to track modals that are currently in a state of transition. When the shown.bs.modal event fires, we remove the modal from the set.

Separately, for removeModal() calls, if we receive a remove modal message that intends to remove a modal currently in the set of transitioning modals, then rather than calling .modal('hide') directly, we instead schedule to it to be called when the shown.bs.modal event occurs. This avoids any kind of race condition and guarantees that the current modal will be hidden as soon as the transition is complete. (From a UI perspective, this also avoids jarring transitions.)

Example app:

library(shiny)

run_model <- function(delay=10.0) {
  start_time <- Sys.time()
  while (difftime(Sys.time(), start_time, units="secs") < delay) {
    # do nothing, just wait
  }
  return(Sys.time())
}

ui <- fluidPage(
  theme = bslib::bs_theme(
    version = 5,
    preset = "shiny",
    # The default transition is 0.3s, try increasing to 3s
    "modal-transition" = "transform 0.3s ease-out"
  ),
  actionButton("run", "Run Model"),
  textOutput("results")
)

server <- function(input, output, session) {
  v_results <- reactiveVal()

  observeEvent(input$run, {
    showModal(modalDialog(
      title = "Running model",
      "The model is running, please wait.",
      footer = NULL,
      easyClose = FALSE
    ))

    # adjust the delay in seconds down below the modal transition time (above)
    results <- run_model(delay=1)

    v_results(results)
    removeModal()
  })

  output$results <- renderText({
    v_results()
  })
}

shinyApp(ui, server)

[schloerke]

LGTM given the two calls are made at the same time, removing the WeakSet usage.