R Shiny laying out 2x2 table in sidebar so that numeric inputs are visible

I've been working on a shiny interface to the bootLR R package, and have it working reasonably well. The interface, however, doesn't seem to show the numbers typed into the numericInput boxes when laid out in a 2x2 table using splitLayout. I've tested this on OS/X and XFCE/Ubuntu/Linux at various resolutions and the problem is the same. I've tried turning the labels off and on, tried different widths for the columns, etc. to no avail.

No matter what I do, it seems to look like this:

I've typed some numbers into the boxes, but the numbers typed are not visible.

How do I fix this layout issue? Should I be using something other than splitLayout?

A minimal reproducible example is below. The web browser document tree looks like this:

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

  # Application title
  titlePanel("Diagnostic test statistics from a 2x2 table"),

  # Sidebar with a slider input for number of digits to round 
  sidebarLayout( fluid = TRUE, 
                 sidebarPanel(
                   fluidRow(
                                splitLayout(
                                  column(6,
                                         numericInput("truePos", "True positive", value = NA),
                                         numericInput("falseNeg", "False negative", value = NA)
                                  ),
                                  column(6,
                                         numericInput("falsePos", "False positive", value = NA),
                                         numericInput("trueNeg", "True negative", value = NA)
                                  )
                                )
                   )
                 ),

                 mainPanel(
                 )
  )
)

# Define server logic required to draw a histogram
server <- function(input, output) {

}

# Run the application 
shinyApp(ui = ui, server = server)