feat(ui): show a tooltip with the full text on truncated titles

Add an `overflowTooltip` Svelte action that sets a native `title` only when
an element is actually clipped (ellipsis or line-clamp), re-checking on
resize. Native title is keyboard/touch reachable and zero-dep, unlike a
hover-only popover. Applied to the clipped titles on MangaCard, BookmarkList,
HistoryList, and the Continue-reading shelf.

Unit tests cover the overflow decision and the set/clear/update behaviour;
an e2e verifies a real-browser truncated card title gets the tooltip while a
short one does not.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/actions/overflowTooltip.test.ts

@@ -0,0 +1,60 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { isOverflowing, overflowTooltip } from './overflowTooltip';
+
+// jsdom doesn't lay out, so scrollWidth/clientWidth are always 0. Stub them
+// to model a clipped vs fitting element.
+function makeEl(dims: {
+    scrollWidth?: number;
+    clientWidth?: number;
+    scrollHeight?: number;
+    clientHeight?: number;
+}): HTMLElement {
+    const el = document.createElement('span');
+    for (const [k, v] of Object.entries(dims)) {
+        Object.defineProperty(el, k, { configurable: true, value: v });
+    }
+    document.body.appendChild(el);
+    return el;
+}
+
+afterEach(() => {
+    document.body.innerHTML = '';
+});
+
+describe('isOverflowing', () => {
+    it('is true when content is clipped horizontally', () => {
+        expect(isOverflowing(makeEl({ scrollWidth: 200, clientWidth: 100 }))).toBe(true);
+    });
+
+    it('is true when content is clipped vertically (line-clamp)', () => {
+        expect(
+            isOverflowing(makeEl({ scrollWidth: 100, clientWidth: 100, scrollHeight: 80, clientHeight: 40 }))
+        ).toBe(true);
+    });
+
+    it('is false when the content fits', () => {
+        expect(isOverflowing(makeEl({ scrollWidth: 100, clientWidth: 100 }))).toBe(false);
+    });
+});
+
+describe('overflowTooltip action', () => {
+    it('sets a title with the text when the element is truncated', () => {
+        const el = makeEl({ scrollWidth: 200, clientWidth: 100 });
+        overflowTooltip(el, 'A very long manga title');
+        expect(el.getAttribute('title')).toBe('A very long manga title');
+    });
+
+    it('does not set a title when the text fits', () => {
+        const el = makeEl({ scrollWidth: 100, clientWidth: 100 });
+        overflowTooltip(el, 'Short');
+        expect(el.getAttribute('title')).toBeNull();
+    });
+
+    it('updates the title text when the bound value changes', () => {
+        const el = makeEl({ scrollWidth: 200, clientWidth: 100 });
+        const action = overflowTooltip(el, 'First');
+        expect(el.getAttribute('title')).toBe('First');
+        action.update('Second');
+        expect(el.getAttribute('title')).toBe('Second');
+    });
+});

frontend/src/lib/actions/overflowTooltip.ts

@@ -0,0 +1,45 @@
+// Svelte action: show the full text in a native `title` tooltip only when
+// the element is actually truncated (ellipsis or line-clamp), and keep it in
+// sync as the element resizes or the text changes.
+//
+// Native `title` is deliberate: it's keyboard/touch reachable and zero-dep,
+// unlike a hover-only custom popover. Applied to clipped titles across list
+// surfaces (MangaCard, BookmarkList, HistoryList).
+
+/** True when the element's content is clipped horizontally (ellipsis) or
+ *  vertically (line-clamp). A 1px slack absorbs sub-pixel rounding. */
+export function isOverflowing(node: HTMLElement): boolean {
+    return (
+        node.scrollWidth - node.clientWidth > 1 ||
+        node.scrollHeight - node.clientHeight > 1
+    );
+}
+
+export function overflowTooltip(node: HTMLElement, text: string) {
+    let current = text;
+
+    function sync() {
+        if (isOverflowing(node)) node.setAttribute('title', current);
+        else node.removeAttribute('title');
+    }
+
+    // Re-check on size changes (viewport resize, font load, layout shifts).
+    // Guarded for environments without ResizeObserver (SSR / jsdom).
+    let ro: ResizeObserver | undefined;
+    if (typeof ResizeObserver !== 'undefined') {
+        ro = new ResizeObserver(() => sync());
+        ro.observe(node);
+    }
+
+    sync();
+
+    return {
+        update(next: string) {
+            current = next;
+            sync();
+        },
+        destroy() {
+            ro?.disconnect();
+        }
+    };
+}