fix(showcase): tear down modal listeners

tests/frontend/components/showcase.listenerMetrics.test.js

@@ -0,0 +1,72 @@
+import { describe, it, beforeEach, afterEach, expect } from 'vitest';
+
+const { SHOWCASE_MODULE } = vi.hoisted(() => ({
+    SHOWCASE_MODULE: new URL('../../../static/js/components/shared/showcase/ShowcaseView.js', import.meta.url).pathname,
+}));
+
+describe('Showcase listener metrics', () => {
+    beforeEach(() => {
+        document.body.innerHTML = `
+            <div id="modelModal">
+                <div class="modal-content">
+                    <div class="showcase-section">
+                        <div class="carousel collapsed">
+                            <div class="scroll-indicator"></div>
+                        </div>
+                        <button class="back-to-top"></button>
+                    </div>
+                </div>
+            </div>
+        `;
+    });
+
+    afterEach(() => {
+        document.body.innerHTML = '';
+    });
+
+    it('tracks wheel/mutation/back-to-top listeners and resets after cleanup', async () => {
+        const {
+            setupShowcaseScroll,
+            resetShowcaseListenerMetrics,
+            showcaseListenerMetrics,
+        } = await import(SHOWCASE_MODULE);
+
+        resetShowcaseListenerMetrics();
+
+        expect(showcaseListenerMetrics.wheelListeners).toBe(0);
+        expect(showcaseListenerMetrics.mutationObservers).toBe(0);
+        expect(showcaseListenerMetrics.backToTopHandlers).toBe(0);
+
+        const cleanup = setupShowcaseScroll('modelModal');
+
+        expect(showcaseListenerMetrics.wheelListeners).toBe(1);
+        expect(showcaseListenerMetrics.mutationObservers).toBe(1);
+        expect(showcaseListenerMetrics.backToTopHandlers).toBe(1);
+
+        cleanup();
+
+        expect(showcaseListenerMetrics.wheelListeners).toBe(0);
+        expect(showcaseListenerMetrics.mutationObservers).toBe(0);
+        expect(showcaseListenerMetrics.backToTopHandlers).toBe(0);
+    });
+
+    it('remains stable after repeated setup/cleanup cycles', async () => {
+        const {
+            setupShowcaseScroll,
+            resetShowcaseListenerMetrics,
+            showcaseListenerMetrics,
+        } = await import(SHOWCASE_MODULE);
+
+        resetShowcaseListenerMetrics();
+
+        const cleanupA = setupShowcaseScroll('modelModal');
+        cleanupA();
+
+        const cleanupB = setupShowcaseScroll('modelModal');
+        cleanupB();
+
+        expect(showcaseListenerMetrics.wheelListeners).toBe(0);
+        expect(showcaseListenerMetrics.mutationObservers).toBe(0);
+        expect(showcaseListenerMetrics.backToTopHandlers).toBe(0);
+    });
+});