In running the below MWE R Shiny code the sidebar panel width correctly remains fixed while the main panel expands as the user expands the size of the window. When the user increases the window size, the size of the plot also increases which is correct. However, when the user decreases the size of the window after having increased it, the plot size does not readjust to the smaller window size: the plot size remains the same large size it was when the window size was increased. How can the plot size be readjusted when the user reduces the window size? While maintaining the same fixed-width sidebar panel.

This automatic resizing works fine when using the standard sidebarPanel() and mainPanel() functions of Shiny, but I wasn’t able to fix the size of the sidebar panel width using these standard functions. The below MWE code correctly fixes the sidebar panel width.

MWE code:

library(ggplot2)
library(shiny)

ui <- fluidPage(
  titlePanel("Title goes here"),
  div(
    style = "display: flex; align-items: flex-start;",
    div(
      class = "sidebar",
      style = "flex-shrink: 0; max-width: 280px; min-width: 280px;",
      wellPanel(
        h2("Sidebar Panel"),
        selectInput("species", "Select Species:", 
                    choices = c("All", unique(as.character(iris$Species))),
                    selected = "All")
      )
    ),
    div(
      class = "main-panel",
      style = "flex-grow: 1; margin-left: 20px;",  # main panel left margin
      h2("Main Panel"),
      plotOutput("plot")
    )
  )
)

server <- function(input, output) {
  output$plot <- renderPlot({
    data <- if (input$species == "All") {
      iris
    } else {
      subset(iris, Species == input$species)
    }
    
    ggplot(data, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
      geom_point() +
      labs(title = "Sepal Length vs Sepal Width",
           x = "Sepal Length",
           y = "Sepal Width")
  })
}

shinyApp(ui = ui, server = server)