DEV: Introduce dynamic wrapper attributes in RenderGlimmer (#22991)

app/assets/javascripts/discourse/app/components/mount-widget.hbs

@@ -1,5 +1,8 @@
 {{#each this._childComponents as |info|}}
   {{#in-element info.element insertBefore=null}}
-    <info.component @data={{info.data}} />
+    <info.component
+      @data={{info.data}}
+      @setWrapperElementAttrs={{info.setWrapperElementAttrs}}
+    />
   {{/in-element}}
 {{/each}}

app/assets/javascripts/discourse/app/widgets/render-glimmer.js

@@ -4,6 +4,8 @@ import { tracked } from "@glimmer/tracking";
 import { assert } from "@ember/debug";
 import { createWidgetFrom } from "discourse/widgets/widget";
 
+const INITIAL_CLASSES = Symbol("RENDER_GLIMMER_INITIAL_CLASSES");
+
 /*
 
 This class allows you to render arbitrary Glimmer templates inside widgets.
@@ -71,8 +73,14 @@ createWidget("my-widget", {
     this.scheduleRerender();
   },
 });
-```
 
+To dynamically control the attributes of the wrapper element, a helper function is provided as an argument to your hbs template.
+To use this via a template, you can do something like this:
+```
+hbs`{{@setWrapperElementAttrs class="some class value" title="title value"}}`
+```
+If you prefer, you can pass this function down into your own components, and call it from there. Invoked as a helper, this can
+be passed (auto-)tracked values, and will update the wrapper element attributes whenever the inputs.
 */
 
 export default class RenderGlimmer {
@@ -104,6 +112,7 @@ export default class RenderGlimmer {
       const [type, ...classNames] = this.renderInto.split(".");
       this.element = document.createElement(type);
       this.element.classList.add(...classNames);
+      this.element[INITIAL_CLASSES] = classNames;
     }
     this.connectComponent();
     return this.element;
@@ -147,11 +156,27 @@ export default class RenderGlimmer {
       element,
       component,
       @tracked data: this.data,
+      setWrapperElementAttrs: (attrs) =>
+        this.updateElementAttrs(element, attrs),
     };
 
     this.parentMountWidgetComponent.mountChildComponent(this._componentInfo);
   }
 
+  updateElementAttrs(element, attrs) {
+    for (let [key, value] of Object.entries(attrs)) {
+      if (key === "class") {
+        value = [element[INITIAL_CLASSES], value].filter(Boolean).join(" ");
+      }
+
+      if ([null, undefined].includes(value)) {
+        element.removeAttribute(key);
+      } else {
+        element.setAttribute(key, value);
+      }
+    }
+  }
+
   get parentMountWidgetComponent() {
     return this.widget?._findView() || this._emberView;
   }

app/assets/javascripts/discourse/tests/integration/components/widgets/render-glimmer-test.js

@@ -133,6 +133,11 @@ module("Integration | Component | Widget | render-glimmer", function (hooks) {
       "div.my-wrapper",
       hbs`<span class='shim-content'>{{@data.attr1}}</span>`
     );
+    registerWidgetShim(
+      "render-glimmer-test-wrapper-attrs",
+      "div.initial-wrapper-class",
+      hbs`{{@setWrapperElementAttrs class=(concat-class "static-extra-class" @data.extraClass) data-some-attr=@data.dataAttrValue}}`
+    );
   });
 
   hooks.afterEach(function () {
@@ -140,6 +145,7 @@ module("Integration | Component | Widget | render-glimmer", function (hooks) {
     this.registry.unregister("widget:toggle-demo-widget");
     this.registry.unregister("component:demo-component");
     deleteFromRegistry("render-glimmer-test-shim");
+    deleteFromRegistry("render-glimmer-test-wrapper-attrs");
   });
 
   test("argument handling", async function (assert) {
@@ -315,4 +321,31 @@ module("Integration | Component | Widget | render-glimmer", function (hooks) {
     assert.dom("div.my-wrapper span.shim-content").exists();
     assert.dom("div.my-wrapper span.shim-content").hasText("val1");
   });
+
+  test("setWrapperElementAttrs API", async function (assert) {
+    await render(
+      hbs`<MountWidget @widget="render-glimmer-test-wrapper-attrs" @args={{hash extraClass=this.extraClass dataAttrValue=this.dataAttrValue}} />`
+    );
+
+    assert.dom("div.initial-wrapper-class").exists();
+    assert
+      .dom("div.initial-wrapper-class")
+      .hasAttribute("class", "initial-wrapper-class static-extra-class");
+    assert
+      .dom("div.initial-wrapper-class")
+      .doesNotHaveAttribute("data-some-attr");
+
+    this.set("extraClass", "dynamic-extra-class");
+    this.set("dataAttrValue", "hello world");
+
+    assert
+      .dom("div.initial-wrapper-class")
+      .hasAttribute(
+        "class",
+        "initial-wrapper-class static-extra-class dynamic-extra-class"
+      );
+    assert
+      .dom("div.initial-wrapper-class")
+      .hasAttribute("data-some-attr", "hello world");
+  });
 });