Elements Don't Re-size Inside tabPanel to Container's Width

[ghost]

I cannot get the elements inside a tabPanel inside a tabBox inside a column inside a fluidRow to re-size to the container's width. MWE below. I'm using shinydashboard_0.7.1.

## load libraries
library(shiny)
library(shinydashboard)

## UI
header = dashboardHeader(title = 'Minimum Working Example', titleWidth = 300)
body = dashboardBody(
    fluidRow(

        column(width = 4,

            tabBox(

                width = NULL,

                tabPanel('Example tab',

                    ## Input: file upload control
                    fileInput('inputFile', 'Choose a CSV file', 
                        accept = c('text/csv', 'text/comma-separated-values, text/plain', '.csv')
                    ),

                    ## Some text inputs
                    textInput('example1', 'Example 1:', value = 'What'),
                    textInput('example2', 'Example 2:', value = 'is'),
                    textInput('example3', 'Example 3:', value = 'going'),
                    textInput('example4', 'Example 4:', value = 'on?')

                )

            )

        ),

        column(width = 8,

            box(title = 'Some content', width = NULL, solidHeader = TRUE, status = 'primary')

        )

    )
)
UI = dashboardPage(header, dashboardSidebar(disable = TRUE), body)

## server
server = function(input, output, session) { }

## run app
shinyApp(UI, server)