Make plotting example work in R CMD check

And provide more informative error if display list is not enabled.

R/content.R

@@ -22,6 +22,7 @@
 #'   content_image_file(system.file("httr2.png", package = "elmer"))
 #' )
 #'
+#' \dontshow{dev.control('enable')}
 #' plot(waiting ~ eruptions, data = faithful)
 #' chat <- new_chat_openai(echo = TRUE)
 #' chat$chat(
@@ -120,6 +121,14 @@ content_image_file <- function(path, content_type = "auto", resize = "low") {
 #' @param width,height Width and height in pixels.
 content_image_plot <- function(width = 768, height = 768) {
   plot <- grDevices::recordPlot()
+
+  if (is.null(plot[[1]])) {
+    cli::cli_abort(c(
+      "Can't record plot because display list is inhibited.",
+      i = "Turn it on with {.code dev.control('enable')}."
+    ))
+  }
+
   old <- grDevices::dev.cur()
 
   path <- tempfile("elmer-plot-", fileext = ".png")

tests/testthat/_snaps/content.md

@@ -56,3 +56,12 @@
       Error:
       ! Invalid geometry string: blah
 
+# useful errors if no display
+
+    Code
+      content_image_plot()
+    Condition
+      Error in `content_image_plot()`:
+      ! Can't record plot because display list is inhibited.
+      i Turn it on with `dev.control('enable')`.
+

tests/testthat/test-content.R

@@ -89,3 +89,9 @@ test_that("image resizing", {
   expect_no_error(content_image_file(img_file, resize = "100x100"))
   expect_no_error(content_image_file(img_file, resize = "100x100>!"))
 })
+
+test_that("useful errors if no display", {
+  # file based devices have display list inhibited
+  withr::local_pdf(NULL)
+  expect_snapshot(content_image_plot(), error = TRUE)
+})