FEATURE: add html-block rich editor extension

app/assets/javascripts/discourse/app/static/prosemirror/extensions/html-block.js

@@ -0,0 +1,35 @@
+/** @type {RichEditorExtension} */
+const extension = {
+  nodeSpec: {
+    html_block: {
+      attrs: { params: { default: "html" } },
+      group: "block",
+      content: "text*",
+      code: true,
+      defining: true,
+      marks: "",
+      isolating: true,
+      selectable: true,
+      draggable: true,
+      parseDOM: [{ tag: "pre.html-block", preserveWhitespace: "full" }],
+      toDOM() {
+        return ["pre", { class: "html-block" }, ["code", 0]];
+      },
+    },
+  },
+  parse: {
+    html_block: (state, token) => {
+      state.openNode(state.schema.nodes.html_block);
+      state.addText(token.content.trim());
+      state.closeNode();
+    },
+  },
+  serializeNode: {
+    html_block: (state, node) => {
+      state.renderContent(node);
+      state.write("\n\n");
+    },
+  },
+};
+
+export default extension;

app/assets/javascripts/discourse/app/static/prosemirror/extensions/register-default.js

@@ -1,4 +1,5 @@
 import { registerRichEditorExtension } from "discourse/lib/composer/rich-editor-extensions";
+import htmlBlock from "./html-block";
 
 /**
  * List of default extensions
@@ -6,6 +7,6 @@ import { registerRichEditorExtension } from "discourse/lib/composer/rich-editor-
  *
  * @type {RichEditorExtension[]}
  */
-const defaultExtensions = [];
+const defaultExtensions = [htmlBlock];
 
 defaultExtensions.forEach(registerRichEditorExtension);