FEATURE: Allow multiple html builders to be registered via plugins

app/controllers/application_controller.rb

@@ -465,9 +465,9 @@ class ApplicationController < ActionController::Base
         data.merge! DiscoursePluginRegistry.custom_html
       end
 
-      DiscoursePluginRegistry.html_builders.each do |name, blk|
+      DiscoursePluginRegistry.html_builders.each do |name, _|
         if name.start_with?("client:")
-          data[name.sub(/^client:/, '')] = blk.call(self)
+          data[name.sub(/^client:/, '')] = DiscoursePluginRegistry.build_html(name, self)
         end
       end
 

lib/discourse_plugin_registry.rb

@@ -137,11 +137,13 @@ class DiscoursePluginRegistry
   end
 
   def self.register_html_builder(name, &block)
-    html_builders[name] = block
+    html_builders[name] ||= []
+    html_builders[name] << block
   end
 
   def self.build_html(name, ctx = nil)
-    html_builders[name]&.call(ctx)
+    builders = html_builders[name] || []
+    builders.map { |b| b.call(ctx) }.join("\n")
   end
 
   def javascripts

spec/components/discourse_plugin_registry_spec.rb

@@ -51,6 +51,13 @@ describe DiscoursePluginRegistry do
       DiscoursePluginRegistry.reset!
       expect(DiscoursePluginRegistry.build_html(:my_html)).to be_blank
     end
+
+    it "can register multiple builders" do
+      DiscoursePluginRegistry.register_html_builder(:my_html) { "one" }
+      DiscoursePluginRegistry.register_html_builder(:my_html) { "two" }
+      expect(DiscoursePluginRegistry.build_html(:my_html)).to eq("one\ntwo")
+      DiscoursePluginRegistry.reset!
+    end
   end
 
   context '.register_css' do