FEATURE: Create IE Support Plugin (#8520)

This core plugin, which could be split off in the future, allows us to load IE specific code on demand.

Co-authored-by: jjaffeux <j.jaffeux@gmail.com>

plugins/discourse-internet-explorer/plugin.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+# name: discourse-internet-explorer
+# about: Attempts to provide backward support for internt explorer
+# version: 1.0
+# authors: Joffrey Jaffeux, David Taylor, Daniel Waterworth, Robin Ward
+# url: https://github.com/discourse/discourse/tree/master/plugins/discourse-internet-explorer
+
+enabled_site_setting :discourse_internet_explorer_enabled
+hide_plugin if self.respond_to?(:hide_plugin)
+
+register_asset 'stylesheets/ie.scss'
+
+after_initialize do
+
+  # Conditionally load the stylesheet. There is unfortunately no easy way to do this via
+  # Plugin API.
+  reloadable_patch do |plugin|
+    ApplicationHelper.module_eval do
+      alias_method :previous_discourse_stylesheet_link_tag, :discourse_stylesheet_link_tag
+      def discourse_stylesheet_link_tag(name, opts = {})
+
+        if name == 'discourse-internet-explorer'
+          return unless SiteSetting.discourse_internet_explorer_enabled?
+          return unless request.env['HTTP_USER_AGENT'] =~ /MSIE|Trident/
+        end
+
+        previous_discourse_stylesheet_link_tag(name, opts)
+      end
+    end
+  end
+
+  register_anonymous_cache_key(:ie) do
+    unless defined?(@is_ie)
+      session = @env[self.class::RACK_SESSION]
+      # don't initialize params until later
+      # otherwise you get a broken params on the request
+      params = {}
+
+      @is_ie = BrowserDetection.browser(@env[self.class::USER_AGENT]) == :ie
+    end
+
+    @is_ie
+  end
+
+  # not using patch on preload_script as js is fine and we need this file
+  # to be loaded before other files
+  register_html_builder('server:before-script-load') do |controller|
+    if BrowserDetection.browser(controller.request.env['HTTP_USER_AGENT']) == :ie
+      path = controller.helpers.script_asset_path('/plugins/discourse-internet-explorer/js/ie')
+
+      <<~JAVASCRIPT
+        <script src="#{path}"></script>
+      JAVASCRIPT
+    end
+  end
+end