DEV: Rebuild JS app when files change in discourse-markdown-it (#24379)

Based on a workaround shared in embroider-build/embroider#1635 (comment)

app/assets/javascripts/discourse/ember-cli-build.js

@@ -14,6 +14,7 @@ const generateWorkboxTree = require("./lib/workbox-tree-builder");
 const { compatBuild } = require("@embroider/compat");
 const { Webpack } = require("@embroider/webpack");
 const { StatsWriterPlugin } = require("webpack-stats-plugin");
+const withSideWatch = require("./lib/with-side-watch");
 const RawHandlebarsCompiler = require("discourse-hbr/raw-handlebars-compiler");
 
 process.env.BROCCOLI_ENABLED_MEMOIZE = true;
@@ -72,10 +73,10 @@ module.exports = function (defaults) {
         "node_modules/@discourse/backburner.js/dist/named-amd/backburner.js",
     },
 
-    historySupportMiddleware: false,
-
     trees: {
-      app: RawHandlebarsCompiler("app"),
+      app: RawHandlebarsCompiler(
+        withSideWatch("app", { watching: ["../discourse-markdown-it"] })
+      ),
     },
   });
 

app/assets/javascripts/discourse/lib/with-side-watch.js

@@ -0,0 +1,27 @@
+// From https://github.com/cardstack/boxel/blob/d54f834f/packages/host/lib/with-side-watch.js (MIT license)
+
+const mergeTrees = require("broccoli-merge-trees");
+const { WatchedDir } = require("broccoli-source");
+const Plugin = require("broccoli-plugin");
+
+class BroccoliNoOp extends Plugin {
+  constructor(path) {
+    super([new WatchedDir(path)]);
+  }
+  build() {}
+}
+
+/*
+  Doesn't change your actualTree, but causes a rebuild when any of opts.watching
+  trees change.
+
+  This is helpful when your build pipeline doesn't naturally watch some
+  dependencies that you're actively developing. For example, right now
+  @embroider/webpack doesn't rebuild itself when non-ember libraries change.
+*/
+module.exports = function withSideWatch(actualTree, opts) {
+  return mergeTrees([
+    actualTree,
+    ...opts.watching.map((w) => new BroccoliNoOp(w)),
+  ]);
+};