Shiny - Black box appearing when returning NULL to plotOutput

Question

In the following very MWE I get a black filled box where the plot should be instead of nothing when I first run the source.

I say nothing because the table does not (IMO correctly) show until the button is clicked. I expect the same behaviour for a plot i.e. until the button is clicked all that will be on screen is the button. What am I doing wrong?

library(shiny)

myUI = bootstrapPage( 
  actionButton(inputId="gobutton", label="Go"),
  plotOutput("plot"),
  tableOutput("table")
  )

mySERVER = function(input, output) {
  output$plot <- renderPlot({ 
    if(input$gobutton==0) {return(NULL)}
    hist(runif(100))
  })
  output$table <- renderTable({ 
    if(input$gobutton==0) {return(NULL)}
    data.frame(x=runif(10), y=seq(1:10))
  })

}

runApp(list(
  ui = myUI,
  server = mySERVER
  ))

I'm using Firefox and the latest stable R & shiny.

> sessionInfo()
R version 3.0.2 (2013-09-25)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_GB.utf8       LC_NUMERIC=C             
 [3] LC_TIME=en_GB.utf8        LC_COLLATE=en_GB.utf8    
 [5] LC_MONETARY=en_GB.utf8    LC_MESSAGES=en_GB.utf8   
 [7] LC_PAPER=en_GB.utf8       LC_NAME=C                
 [9] LC_ADDRESS=C              LC_TELEPHONE=C           
[11] LC_MEASUREMENT=en_GB.utf8 LC_IDENTIFICATION=C      

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] shiny_0.8.0

loaded via a namespace (and not attached):
[1] bitops_1.0-6  Cairo_1.5-5   caTools_1.16  digest_0.6.4  httpuv_1.2.2 
[6] Rcpp_0.11.0   RJSONIO_1.0-3 tools_3.0.2   xtable_1.7-1

Answer 1

I think this has to do with the Cairo package.

My system did not have Cairo installed. If I run your code in Firefox with a fresh R session, no black box (just the GO button).

I then install and load Cairo. When I re-run your code, I get the black box.

I then unload Cairo detach("package:Cairo", unload=T) : still get black box.

I then restart R and run your code: still black box.

I then ininstall Cairo remove.packages("Cairo") and restart R: no black box.

So it appears the Shiny will load Cairo if it is intstalled, and that is what causes the black box.