Add override to include BOM in CSVs exported by Decidim::Exporters::CSV

app/lib/decidim/exporters/csv_overrides.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module Decidim
+  module Exporters
+    # Overrides Decidim::Exporters::CSV#export to prepend the UTF-8 byte order
+    # mark to the generated CSV, so spreadsheet software (notably Microsoft
+    # Excel) detects the encoding and renders non-ASCII characters correctly.
+    #
+    # Upstream Decidim does not include the BOM and the maintainers won't accept
+    # this change, so it is applied locally as an override instead of forking.
+    module CsvOverrides
+      # UTF-8 byte order mark.
+      BOM = ""
+
+      # Public: Exports a CSV serialized version of the collection, prepending
+      # the UTF-8 BOM to the output.
+      #
+      # Returns an ExportData instance.
+      def export(col_sep = Decidim.default_csv_col_sep)
+        data = ::CSV.generate(BOM.dup, headers:, write_headers: true, col_sep:) do |csv|
+          processed_collection.each do |resource|
+            csv << headers.map { |header| custom_sanitize(resource[header]) }
+          end
+        end
+        ExportData.new(data, "csv")
+      end
+    end
+  end
+end

config/initializers/lib_overrides.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+Rails.application.config.to_prepare do
+  Decidim::Exporters::CSV.prepend(Decidim::Exporters::CsvOverrides)
+end

spec/lib/overrides_spec.rb

@@ -10,7 +10,10 @@
     files: {
       "/app/cells/decidim/newsletter_templates/basic_only_text/show.erb" => "7d9d4f2ab8897143fe66e8ac8db2cedb",
       "/app/cells/decidim/newsletter_templates/basic_only_text_settings_form/show.erb" => "cc8e26ddc53c0b65eddf472ed4c5614e",
-      "/app/cells/decidim/newsletter_templates/image_text_cta/show.erb" => "08881fe2df7d87db0497376b08821594"
+      "/app/cells/decidim/newsletter_templates/image_text_cta/show.erb" => "08881fe2df7d87db0497376b08821594",
+      # Overridden by Decidim::Exporters::CsvOverrides (config/initializers/lib_overrides.rb)
+      # to prepend the UTF-8 BOM. If this checksum changes, review the override.
+      "/lib/decidim/exporters/csv.rb" => "26eeaecb13dbb4dbce28e39e8dc7a3fd"
     }
   }
 ]