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diff --git a/frontend-vanilla/src/core_robustness.test.ts b/frontend-vanilla/src/core_robustness.test.ts
new file mode 100644
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--- /dev/null
+++ b/frontend-vanilla/src/core_robustness.test.ts
@@ -0,0 +1,226 @@
+vi.useFakeTimers();
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { apiFetch } from './api';
+
+// Mock api
+vi.mock('./api', () => ({
+    apiFetch: vi.fn()
+}));
+
+describe('Core Robustness: Scroll-to-Read, Infinite Scroll, Keyboard Nav', () => {
+    let mainContent: HTMLElement;
+    let renderItems: any;
+    let updateItem: any;
+    let storeInstance: any;
+
+    beforeEach(async () => {
+        vi.useFakeTimers();
+        vi.resetModules();
+        const main = await import('./main');
+        const storeMod = await import("./store");
+        storeInstance = storeMod.store;
+        renderItems = main.renderItems;
+        updateItem = main.updateItem;
+
+        document.body.innerHTML = '<div id="app"><div id="main-content"><div id="content-area"></div></div></div>';
+        mainContent = document.getElementById('main-content')!;
+
+        // Mock scrollIntoView
+        Element.prototype.scrollIntoView = vi.fn();
+
+        vi.clearAllMocks();
+
+        // Reset store
+        storeInstance.setItems([]);
+        storeInstance.setHasMore(true);
+        storeInstance.setLoading(false);
+        storeInstance.setSearchQuery('');
+        storeInstance.setFilter('unread');
+
+        // Default successful API mock
+        vi.mocked(apiFetch).mockResolvedValue({
+            ok: true,
+            status: 200,
+            json: async () => []
+        } as Response);
+    });
+
+    afterEach(() => {
+        vi.useRealTimers();
+    });
+
+    // --- SCROLL-TO-READ ROBUSTNESS ---
+
+    it('should mark multiple items as read when fast-scrolled past (debounce check)', async () => {
+        const items = Array.from({ length: 5 }, (_, i) => ({
+            _id: i + 1, title: `Item ${i + 1}`, read: false, publish_date: '2023-01-01'
+        })) as any[];
+        storeInstance.setItems(items);
+        renderItems();
+
+        // Mock container at top 0
+        mainContent.getBoundingClientRect = vi.fn(() => ({ top: 0, bottom: 800 } as any));
+
+        // Mock items scattered past the top
+        document.querySelectorAll('.feed-item').forEach((el, i) => {
+            el.getBoundingClientRect = vi.fn(() => ({
+                top: -100 - (i * 100),
+                bottom: -10 - (i * 100) // All 5 are past the top
+            } as any));
+        });
+
+        // Trigger multiple scroll events rapidly
+        mainContent.dispatchEvent(new Event('scroll'));
+        mainContent.dispatchEvent(new Event('scroll'));
+        mainContent.dispatchEvent(new Event('scroll'));
+
+        // Advance timers by 250ms (debounce)
+        vi.advanceTimersByTime(300);
+
+        // Should have called updateItem for all 5 items
+        // We check apiFetch since updateItem calls it
+        expect(apiFetch).toHaveBeenCalledTimes(5);
+        items.forEach(item => {
+            expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining(`/api/item/${item._id}`), expect.anything());
+        });
+    });
+
+    it('should NOT mark a very tall item as read until its BOTTOM clears the container top', async () => {
+        storeInstance.setItems([{ _id: 1, title: 'Tall Item', read: false, publish_date: '2023-01-01' }] as any);
+        renderItems();
+
+        mainContent.getBoundingClientRect = vi.fn(() => ({ top: 0, bottom: 800 } as any));
+        const itemEl = document.querySelector('.feed-item[data-id="1"]')!;
+
+        // Scenario: Top is way off screen (-500), but bottom is still visible (+10)
+        itemEl.getBoundingClientRect = vi.fn(() => ({ top: -500, bottom: 10 } as any));
+
+        mainContent.dispatchEvent(new Event('scroll'));
+        vi.advanceTimersByTime(300);
+
+        expect(apiFetch).not.toHaveBeenCalled();
+
+        // Scenario: Bottom finally clears (is past top + 5px buffer)
+        // bottom: -10 means it's 10px above the top
+        itemEl.getBoundingClientRect = vi.fn(() => ({ top: -1000, bottom: -10 } as any));
+
+        mainContent.dispatchEvent(new Event('scroll'));
+        vi.advanceTimersByTime(300);
+
+        expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/item/1'), expect.anything());
+    });
+
+    // --- INFINITE SCROLL ROBUSTNESS ---
+
+    it('should handle "stuck" loads by allowing re-trigger if previous load finished', async () => {
+        storeInstance.setItems(Array.from({ length: 10 }, (_, i) => ({ _id: i })) as any);
+        renderItems();
+
+        // Mock being at the bottom
+        Object.defineProperty(mainContent, 'scrollHeight', { value: 1000 });
+        Object.defineProperty(mainContent, 'scrollTop', { value: 850 });
+        Object.defineProperty(mainContent, 'clientHeight', { value: 100 });
+        // scrollHeight(1000) - scrollTop(850) - clientHeight(100) = 50 (< 200)
+
+        // Simulate loadMore takes time
+        let resolvePromise: (v: any) => void;
+        const pendingPromise = new Promise(resolve => { resolvePromise = resolve; });
+        vi.mocked(apiFetch).mockReturnValue(pendingPromise as any);
+
+        mainContent.dispatchEvent(new Event('scroll'));
+        expect(apiFetch).toHaveBeenCalledTimes(1); // First trigger
+        expect(storeInstance.loading).toBe(true);
+
+        // Scroll again while loading
+        mainContent.dispatchEvent(new Event('scroll'));
+        expect(apiFetch).toHaveBeenCalledTimes(1); // Should NOT trigger again while storeInstance.loading is true
+
+        // Finish first load
+        resolvePromise!({ ok: true, json: async () => [] });
+        await vi.runAllTicks();
+        storeInstance.setLoading(false); // Manually set since we're testing the logic flow
+
+        // Scroll again
+        mainContent.dispatchEvent(new Event('scroll'));
+        expect(apiFetch).toHaveBeenCalledTimes(2); // Should trigger again now that loading is false
+    });
+
+    // --- KEYBOARD NAVIGATION ROBUSTNESS ---
+
+    it('should sync activeItemId and scroll into view correctly on j/k', async () => {
+        storeInstance.setItems([
+            { _id: 101, title: 'Item 1', read: false },
+            { _id: 102, title: 'Item 2', read: false }
+        ] as any);
+        renderItems();
+
+        const scrollSpy = vi.spyOn(Element.prototype, 'scrollIntoView');
+
+        // Press 'j'
+        window.dispatchEvent(new KeyboardEvent('keydown', { key: 'j' }));
+        expect(scrollSpy).toHaveBeenCalled();
+        expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/item/101'), expect.objectContaining({
+            body: expect.stringContaining('"read":true')
+        }));
+
+        // Press 'j' again
+        window.dispatchEvent(new KeyboardEvent('keydown', { key: 'j' }));
+        expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/item/102'), expect.anything());
+    });
+
+    // --- API ERROR ENCAPSULATION ---
+
+    it('should NOT update DOM classes if API update fails', async () => {
+        storeInstance.setItems([{ _id: 99, title: 'Error Item', read: false }] as any);
+        renderItems();
+
+        const el = document.querySelector('.feed-item[data-id="99"]')!;
+        expect(el.classList.contains('unread')).toBe(true);
+
+        // Mock API failure
+        vi.mocked(apiFetch).mockResolvedValue({ ok: false, status: 500 } as Response);
+
+        await updateItem(99, { read: true });
+
+        // DOM should NOT be updated because res.ok was false
+        expect(el.classList.contains('read')).toBe(false);
+        expect(el.classList.contains('unread')).toBe(true);
+    });
+
+    // --- POLLING FALLBACK ---
+
+    it('should trigger checkReadItems periodically via polling fallback', async () => {
+        storeInstance.setItems([{ _id: 55, title: 'Poll Item', read: false }] as any);
+        renderItems();
+
+        mainContent.getBoundingClientRect = vi.fn(() => ({ top: 0, bottom: 800 } as any));
+        const itemEl = document.querySelector('.feed-item[data-id="55"]')!;
+        itemEl.getBoundingClientRect = vi.fn(() => ({ top: -100, bottom: -50 } as any));
+
+        // Advance 1s (the polling interval in main.ts)
+        vi.advanceTimersByTime(1100);
+
+        expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/item/55'), expect.anything());
+    });
+
+    // --- EDGE CASE: EMPTY / BOUNDARY NAVIGATION ---
+
+    it('should not crash when j/k is pressed on an empty list', () => {
+        storeInstance.setItems([]);
+        renderItems();
+
+        // Should not throw
+        window.dispatchEvent(new KeyboardEvent('keydown', { key: 'j' }));
+        window.dispatchEvent(new KeyboardEvent('keydown', { key: 'k' }));
+    });
+
+    it('should handle search queries correctly in stream requests', async () => {
+        storeInstance.setSearchQuery('rust');
+        // trigger fetchItems through store logic or manual call
+        const { fetchItems } = await import('./main');
+        await fetchItems();
+
+        expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('q=rust'));
+    });
+});