FIX: Ensure 'modern' modal is closed when opening legacy modal (#22689)

e.g. the modernised share-topic modal will attempt to open the `create-invite` modal. Prior to this commit, this mixing of modern/legacy would fail silently, and the create-invite modal was never shown.

app/assets/javascripts/discourse/app/services/modal.js

@@ -193,6 +193,8 @@ export default class ModalServiceWithLegacySupport extends ModalService {
       return super.show(modal, opts);
     }
 
+    this.close({ initiatedBy: CLOSE_INITIATED_BY_MODAL_SHOW });
+
     if (!KNOWN_LEGACY_MODALS.includes(modal)) {
       deprecated(
         `Defining modals using a controller is deprecated. Use the component-based API instead. (modal: ${modal})`,

app/assets/javascripts/discourse/tests/acceptance/modal-legacy-test.js

@@ -12,6 +12,8 @@ import showModal from "discourse/lib/show-modal";
 import { registerTemporaryModule } from "../helpers/temporary-module-helper";
 import { getOwner } from "discourse-common/lib/get-owner";
 import { withSilencedDeprecations } from "discourse-common/lib/deprecated";
+import Component from "@glimmer/component";
+import { setComponentTemplate } from "@glimmer/manager";
 
 function silencedShowModal() {
   return withSilencedDeprecations("discourse.modal-controllers", () =>
@@ -142,6 +144,32 @@ acceptance("Legacy Modal", function (needs) {
       "it should not re-use the previous title"
     );
   });
+
+  test("opening legacy modal while modern modal is open", async function (assert) {
+    registerTemporaryModule(
+      "discourse/templates/modal/legacy-modal",
+      hbs`<DModalBody @rawTitle="legacy modal title" />`
+    );
+
+    class ModernModal extends Component {}
+    setComponentTemplate(
+      hbs`<DModal @title="modern modal title" />`,
+      ModernModal
+    );
+
+    await visit("/");
+
+    const modalService = getOwner(this).lookup("service:modal");
+
+    modalService.show(ModernModal);
+    await settled();
+    assert.dom(".d-modal .title").hasText("modern modal title");
+
+    silencedShowModal("legacy-modal");
+    await settled();
+
+    assert.dom(".d-modal .title").hasText("legacy modal title");
+  });
 });
 
 acceptance("Modal Keyboard Events", function (needs) {