selectizeInput()'s input value can be incorrect after updateSelectizeInput()

[cpsievert]

Introduced by #3875 and discovered via this test app. Below is a minimal reprex of the problem we were seeing there.

To reproduce, run the app.R below. Once open, remove "A" and "D" from the 1st selectize (choices). That causes input$selected to be NULL (when it should be "B"!).

If you then open the browser's devtools, and run the following JS $("select#selected").val(), you'll see the result is [], which is why the binding is reporting NULL. Interestingly, $("select#selected").selectize().getValue() returns the expect result (["B"]), so selectize.js must've recently moved away from using the DOM as the source of truth for the select's state.

app.R

library(shiny)

choices <- c("A", "B", "C", "D")

ui <- fluidPage(
  verbatimTextOutput("selected"),
  div(
    style = "display:flex;",
    selectInput(
      "choices", NULL, multiple = TRUE,
      choices = choices, selected = choices
    ),
    selectInput(
      "selected", NULL, multiple = TRUE, 
      choices, selected = c("A", "B", "D")
    )
  )
)

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

  observeEvent(input$choices, ignoreInit = TRUE, {
    updateSelectInput(
      inputId = "selected", choices = input$choices,
      selected = input$selected
    )
  })

  output$selected <- renderPrint({
    paste("Value of the 2nd selectize (input$selected):", paste(input$selected, collapse = ", "))
  })

}

shinyApp(ui, server)