With Bootstrap 5, uiOutput() doesn't dim in opacity when recalculating

[cpsievert]

Starting with #3957, uiOutput() (and conditionalPanel()) containers started getting display: contents;. Turns out this is problematic for the .recalculating { opacity: 0.3; } that shiny adds to output containers. Consider the following example which now fails to dim in opacity when recalculating (since the output container div is now display: contents;, opacity no longer applies):

library(shiny)
library(bslib)

ui <- page_fixed(
  actionButton("btn", "Press me"),
  uiOutput("foo")
)

server <- function(input, output, session) {
  
  output$foo <- renderUI({
    input$btn
    Sys.sleep(3)
    span(paste("Number of clicks:", input$btn))
  })
  
}

shinyApp(ui, server)