feat(documents): zoom-in tool for the timeline (#385)

Adds a zoom action that narrows the visible timeline range to the current
selection so the user can drill from year-level back into month-level
density. Zoom state lives in the URL (zoomFrom / zoomTo) so it survives
reload and is shareable.

- New `clipBucketsToRange(buckets, from, to)` helper applied before the
  >240-month year-aggregate decision, so a zoomed window flips back to
  month bars automatically when the clip narrows the range enough.
- `TimelineDensityFilter` gains `zoomFrom`, `zoomTo`, and `onzoomchange`
  props. Zoom button shown only when a selection exists and we aren't
  already zoomed; reset-zoom shown only when zoomed. Both placed in a
  shared right-edge action cluster alongside the × clear button.
- `+page.ts` reads zoomFrom/zoomTo from the URL and forwards them as
  props. `+page.svelte` extends FilterSnapshot + buildSearchParams, and
  triggerSearch accepts an optional zoom override so the onzoomchange
  callback can write the new pair (or clear them) atomically.
- 7 new component tests + 2 new page-integration tests cover the
  visibility rules and URL writes.
- 4 new unit tests for `clipBucketsToRange`.
- 3 new i18n keys (zoom in / zoom reset / drag aria-live) across de/en/es.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1051,5 +1051,8 @@
 	"timeline_clear_selection": "Auswahl zurücksetzen",
 	"timeline_count_label": "Dok.",
 	"timeline_loading": "Lade Zeitachse…",
-	"timeline_filtered_count": "{count} Dokumente im Zeitraum"
+	"timeline_filtered_count": "{count} Dokumente im Zeitraum",
+	"timeline_zoom_in": "Zeitraum vergrößern",
+	"timeline_zoom_reset": "Zurück zur Übersicht",
+	"timeline_dragging_aria_live": "Zeitraum {from} bis {to} ausgewählt"
 }

frontend/messages/en.json

@@ -1051,5 +1051,8 @@
 	"timeline_clear_selection": "Clear selection",
 	"timeline_count_label": "docs",
 	"timeline_loading": "Loading timeline…",
-	"timeline_filtered_count": "{count} documents in range"
+	"timeline_filtered_count": "{count} documents in range",
+	"timeline_zoom_in": "Zoom in",
+	"timeline_zoom_reset": "Reset zoom",
+	"timeline_dragging_aria_live": "Range {from} to {to} selected"
 }

frontend/messages/es.json

@@ -1051,5 +1051,8 @@
 	"timeline_clear_selection": "Borrar selección",
 	"timeline_count_label": "docs",
 	"timeline_loading": "Cargando cronología…",
-	"timeline_filtered_count": "{count} documentos en el rango"
+	"timeline_filtered_count": "{count} documentos en el rango",
+	"timeline_zoom_in": "Acercar",
+	"timeline_zoom_reset": "Restablecer zoom",
+	"timeline_dragging_aria_live": "Rango {from} a {to} seleccionado"
 }

frontend/src/lib/document/TimelineDensityFilter.svelte

@@ -3,6 +3,7 @@ import * as m from '$lib/paraglide/messages.js';
 import {
 	fillDensityGaps,
 	aggregateToYears,
+	clipBucketsToRange,
 	selectionBoundaryFrom,
 	selectionBoundaryTo
 } from '$lib/document/timeline';
@@ -10,6 +11,7 @@ import type { components } from '$lib/generated/api';
 
 type MonthBucket = components['schemas']['MonthBucket'];
 type SelectionEvent = { from: string; to: string };
+type ZoomEvent = { zoomFrom: string; zoomTo: string };
 
 const BAR_AREA_HEIGHT = 80; // px — Leonie spec h-20
 const ZERO_COUNT_BAR_HEIGHT = 2; // px — minimum visible signal for empty months
@@ -23,25 +25,44 @@ let {
 	maxDate,
 	from,
 	to,
-	onchange
+	zoomFrom = null,
+	zoomTo = null,
+	onchange,
+	onzoomchange
 }: {
 	density: MonthBucket[] | null;
 	minDate: string | null;
 	maxDate: string | null;
 	from: string;
 	to: string;
+	zoomFrom?: string | null;
+	zoomTo?: string | null;
 	onchange: (event: SelectionEvent) => void;
+	onzoomchange?: (event: ZoomEvent | null) => void;
 } = $props();
 
 const monthBuckets = $derived.by(() => {
 	if (density === null) return [];
-	return fillDensityGaps(density, minDate, maxDate);
+	const full = fillDensityGaps(density, minDate, maxDate);
+	return clipBucketsToRange(full, zoomFrom, zoomTo);
 });
 
 const filled = $derived(
 	monthBuckets.length > MONTH_GRANULARITY_LIMIT ? aggregateToYears(monthBuckets) : monthBuckets
 );
 
+const isZoomed = $derived(zoomFrom !== null && zoomTo !== null);
+const canZoomIn = $derived(hasSelection && !isZoomed);
+
+function zoomIn() {
+	if (from === '' || to === '') return;
+	onzoomchange?.({ zoomFrom: from, zoomTo: to });
+}
+
+function resetZoom() {
+	onzoomchange?.(null);
+}
+
 const maxCount = $derived(Math.max(...filled.map((b) => b.count), 1));
 
 const hasSelection = $derived(from !== '' || to !== '');
@@ -166,17 +187,43 @@ function handleClick(index: number) {
 			{/each}
 		</div>
 
-		{#if hasSelection}
-			<button
-				type="button"
-				data-testid="timeline-clear"
-				aria-label={m.timeline_clear_selection()}
-				onclick={clearSelection}
-				class="hover:text-ink-1 absolute top-2 right-2 inline-flex h-6 w-6 items-center justify-center rounded-full text-ink-3 hover:bg-canvas"
-			>
-				×
-			</button>
-		{/if}
+		<div class="absolute top-2 right-2 flex items-center gap-1">
+			{#if canZoomIn}
+				<button
+					type="button"
+					data-testid="timeline-zoom-in"
+					aria-label={m.timeline_zoom_in()}
+					title={m.timeline_zoom_in()}
+					onclick={zoomIn}
+					class="hover:text-ink-1 inline-flex h-6 items-center justify-center gap-1 rounded-sm px-2 text-xs text-ink-3 hover:bg-canvas"
+				>
+					🔍
+				</button>
+			{/if}
+			{#if isZoomed}
+				<button
+					type="button"
+					data-testid="timeline-zoom-reset"
+					aria-label={m.timeline_zoom_reset()}
+					title={m.timeline_zoom_reset()}
+					onclick={resetZoom}
+					class="hover:text-ink-1 inline-flex h-6 items-center justify-center gap-1 rounded-sm px-2 text-xs text-ink-3 hover:bg-canvas"
+				>
+					↩
+				</button>
+			{/if}
+			{#if hasSelection}
+				<button
+					type="button"
+					data-testid="timeline-clear"
+					aria-label={m.timeline_clear_selection()}
+					onclick={clearSelection}
+					class="hover:text-ink-1 inline-flex h-6 w-6 items-center justify-center rounded-full text-ink-3 hover:bg-canvas"
+				>
+					×
+				</button>
+			{/if}
+		</div>
 	</div>
 {/if}
 

frontend/src/lib/document/TimelineDensityFilter.svelte.spec.ts

@@ -175,6 +175,95 @@ describe('TimelineDensityFilter — year-granularity fallback', () => {
 	});
 });
 
+describe('TimelineDensityFilter — zoom-in', () => {
+	it('does not show the zoom button when there is no selection', async () => {
+		render(TimelineDensityFilter, makeProps());
+		expect(document.querySelector('[data-testid="timeline-zoom-in"]')).toBeNull();
+	});
+
+	it('shows the zoom button when from/to are set and not yet zoomed', async () => {
+		render(TimelineDensityFilter, makeProps({ from: '1915-08-01', to: '1915-09-30' }));
+		await expect.element(page.getByTestId('timeline-zoom-in')).toBeInTheDocument();
+	});
+
+	it('hides the zoom button when zoomFrom/zoomTo are already set', async () => {
+		render(
+			TimelineDensityFilter,
+			makeProps({
+				from: '1915-08-01',
+				to: '1915-09-30',
+				zoomFrom: '1915-08-01',
+				zoomTo: '1915-09-30'
+			})
+		);
+		expect(document.querySelector('[data-testid="timeline-zoom-in"]')).toBeNull();
+	});
+
+	it('clicking the zoom button emits onzoomchange with the current selection', async () => {
+		const onzoomchange = vi.fn();
+		render(
+			TimelineDensityFilter,
+			makeProps({ from: '1915-08-01', to: '1915-09-30', onzoomchange })
+		);
+
+		const zoomBtn = document.querySelector('[data-testid="timeline-zoom-in"]') as HTMLButtonElement;
+		zoomBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(onzoomchange).toHaveBeenCalledWith({ zoomFrom: '1915-08-01', zoomTo: '1915-09-30' });
+	});
+
+	it('shows the reset-zoom button only when zoomed', async () => {
+		render(TimelineDensityFilter, makeProps({ zoomFrom: '1915-08-01', zoomTo: '1915-09-30' }));
+		await expect.element(page.getByTestId('timeline-zoom-reset')).toBeInTheDocument();
+	});
+
+	it('clicking reset-zoom emits onzoomchange(null)', async () => {
+		const onzoomchange = vi.fn();
+		render(
+			TimelineDensityFilter,
+			makeProps({ zoomFrom: '1915-08-01', zoomTo: '1915-09-30', onzoomchange })
+		);
+
+		const resetBtn = document.querySelector(
+			'[data-testid="timeline-zoom-reset"]'
+		) as HTMLButtonElement;
+		resetBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(onzoomchange).toHaveBeenCalledWith(null);
+	});
+
+	it('when zoomed, only bars within the zoom range are rendered', async () => {
+		// 21-year span normally collapses to year mode (>240 months handled
+		// elsewhere). Zooming in to a 3-month window should restore month bars.
+		const buckets: MonthBucket[] = [];
+		for (let year = 1900; year <= 1920; year++) {
+			for (let month = 1; month <= 12; month++) {
+				buckets.push({
+					month: `${year}-${String(month).padStart(2, '0')}`,
+					count: 1
+				});
+			}
+		}
+
+		render(
+			TimelineDensityFilter,
+			makeProps({
+				density: buckets,
+				minDate: '1900-01-01',
+				maxDate: '1920-12-31',
+				zoomFrom: '1910-06-01',
+				zoomTo: '1910-08-31'
+			})
+		);
+
+		const bars = document.querySelectorAll('[data-testid="timeline-bar"]');
+		// 3 months in zoom range
+		expect(bars.length).toBe(3);
+		expect(bars[0].getAttribute('aria-label')?.startsWith('1910-06 ·')).toBe(true);
+		expect(bars[2].getAttribute('aria-label')?.startsWith('1910-08 ·')).toBe(true);
+	});
+});
+
 describe('TimelineDensityFilter — drag-to-select-range', () => {
 	function pointerDown(el: HTMLElement) {
 		const event = new PointerEvent('pointerdown', { bubbles: true, pointerId: 1, button: 0 });

frontend/src/lib/document/timeline.spec.ts

@@ -8,7 +8,8 @@ import {
 	buildDensityUrl,
 	aggregateToYears,
 	selectionBoundaryFrom,
-	selectionBoundaryTo
+	selectionBoundaryTo,
+	clipBucketsToRange
 } from './timeline';
 
 describe('monthBoundaryFrom', () => {
@@ -141,6 +142,37 @@ describe('aggregateToYears', () => {
 	});
 });
 
+describe('clipBucketsToRange', () => {
+	const buckets = [
+		{ month: '1915-08', count: 5 },
+		{ month: '1915-09', count: 2 },
+		{ month: '1915-10', count: 8 },
+		{ month: '1915-11', count: 3 }
+	];
+
+	it('returns the original buckets when range bounds are null', () => {
+		expect(clipBucketsToRange(buckets, null, null)).toBe(buckets);
+	});
+
+	it('keeps only buckets whose month falls within the range', () => {
+		expect(clipBucketsToRange(buckets, '1915-09-01', '1915-10-31')).toEqual([
+			{ month: '1915-09', count: 2 },
+			{ month: '1915-10', count: 8 }
+		]);
+	});
+
+	it('returns an empty array when the range excludes everything', () => {
+		expect(clipBucketsToRange(buckets, '1916-01-01', '1916-12-31')).toEqual([]);
+	});
+
+	it('treats partial dates correctly when bounds cross month boundaries', () => {
+		expect(clipBucketsToRange(buckets, '1915-09-15', '1915-10-15')).toEqual([
+			{ month: '1915-09', count: 2 },
+			{ month: '1915-10', count: 8 }
+		]);
+	});
+});
+
 describe('selectionBoundaryFrom / To', () => {
 	it('handles month labels (YYYY-MM)', () => {
 		expect(selectionBoundaryFrom('1915-08')).toBe('1915-08-01');

frontend/src/lib/document/timeline.ts

@@ -56,6 +56,23 @@ export function fillDensityGaps(
 	return sequence.map((month) => ({ month, count: counts.get(month) ?? 0 }));
 }
 
+/**
+ * Returns only the month buckets whose YYYY-MM falls inside the provided
+ * `[fromInclusive, toInclusive]` ISO date range. When either bound is null the
+ * input array is returned unchanged. Used by the timeline's zoom-in tool to
+ * narrow the visible bars without refetching data.
+ */
+export function clipBucketsToRange(
+	buckets: MonthBucket[],
+	fromInclusive: string | null,
+	toInclusive: string | null
+): MonthBucket[] {
+	if (!fromInclusive || !toInclusive) return buckets;
+	const fromMonth = fromInclusive.slice(0, 7);
+	const toMonth = toInclusive.slice(0, 7);
+	return buckets.filter((b) => b.month >= fromMonth && b.month <= toMonth);
+}
+
 /**
  * Aggregates month-granular buckets into one entry per year. Month strings are
  * truncated to "YYYY" and counts are summed. Used when the date span is too

frontend/src/routes/documents/+page.svelte

@@ -51,6 +51,8 @@ type FilterSnapshot = {
 	dir: string;
 	tagQ: string;
 	tagOp: 'AND' | 'OR';
+	zoomFrom?: string | null;
+	zoomTo?: string | null;
 };
 
 /**
@@ -72,6 +74,8 @@ function buildSearchParams(filters: FilterSnapshot, targetPage?: number): Svelte
 	if (filters.dir) params.set('dir', filters.dir);
 	if (filters.tagQ) params.set('tagQ', filters.tagQ);
 	if (filters.tagOp === 'OR') params.set('tagOp', 'OR');
+	if (filters.zoomFrom) params.set('zoomFrom', filters.zoomFrom);
+	if (filters.zoomTo) params.set('zoomTo', filters.zoomTo);
 	if (targetPage !== undefined && targetPage > 0) params.set('page', String(targetPage));
 	return params;
 }
@@ -80,7 +84,7 @@ function buildSearchParams(filters: FilterSnapshot, targetPage?: number): Svelte
  * Rebuilds the URL from the CURRENT local filter state. `page` is intentionally
  * not carried over — any filter change implicitly resets back to page 0.
  */
-function triggerSearch() {
+function triggerSearch(zoomOverride?: { zoomFrom: string | null; zoomTo: string | null }) {
 	const params = buildSearchParams({
 		q,
 		from,
@@ -91,7 +95,9 @@ function triggerSearch() {
 		sort,
 		dir,
 		tagQ,
-		tagOp: tagOperator
+		tagOp: tagOperator,
+		zoomFrom: zoomOverride ? zoomOverride.zoomFrom : data.zoomFrom,
+		zoomTo: zoomOverride ? zoomOverride.zoomTo : data.zoomTo
 	});
 	goto(`/documents?${params.toString()}`, { keepFocus: true, noScroll: true });
 }
@@ -240,6 +246,8 @@ $effect(() => {
 			density={data.density}
 			minDate={data.minDate}
 			maxDate={data.maxDate}
+			zoomFrom={data.zoomFrom}
+			zoomTo={data.zoomTo}
 			from={from}
 			to={to}
 			onchange={(event) => {
@@ -247,6 +255,12 @@ $effect(() => {
 				to = event.to;
 				triggerSearch();
 			}}
+			onzoomchange={(event) => {
+				triggerSearch({
+					zoomFrom: event?.zoomFrom ?? null,
+					zoomTo: event?.zoomTo ?? null
+				});
+			}}
 		/>
 	</div>
 

frontend/src/routes/documents/+page.ts

@@ -19,5 +19,8 @@ export const load: PageLoad = async ({ url, fetch, data }) => {
 	};
 
 	const density = await fetchDensity(fetch, view, isDesktop, filters);
-	return { ...data, ...density };
+	const zoomFrom = url.searchParams.get('zoomFrom');
+	const zoomTo = url.searchParams.get('zoomTo');
+
+	return { ...data, ...density, zoomFrom, zoomTo };
 };

frontend/src/routes/documents/page.svelte.spec.ts

@@ -176,4 +176,55 @@ describe('documents page — timeline density widget', () => {
 		expect(url).toContain('from=1915-08-01');
 		expect(url).toContain('to=1915-08-31');
 	});
+
+	it('clicking the zoom-in button writes zoomFrom/zoomTo URL params', async () => {
+		const { goto } = await import('$app/navigation');
+		vi.mocked(goto).mockClear();
+
+		render(Page, {
+			data: makeData({
+				density: [
+					{ month: '1915-08', count: 3 },
+					{ month: '1915-09', count: 2 }
+				],
+				minDate: '1915-08-01',
+				maxDate: '1915-09-30',
+				from: '1915-08-01',
+				to: '1915-09-30'
+			})
+		});
+
+		const zoomBtn = document.querySelector('[data-testid="timeline-zoom-in"]') as HTMLButtonElement;
+		zoomBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(goto).toHaveBeenCalledOnce();
+		const [url] = vi.mocked(goto).mock.calls[0];
+		expect(url).toContain('zoomFrom=1915-08-01');
+		expect(url).toContain('zoomTo=1915-09-30');
+	});
+
+	it('clicking reset-zoom drops zoomFrom/zoomTo from the URL', async () => {
+		const { goto } = await import('$app/navigation');
+		vi.mocked(goto).mockClear();
+
+		render(Page, {
+			data: makeData({
+				density: [{ month: '1915-08', count: 3 }],
+				minDate: '1915-08-01',
+				maxDate: '1915-08-31',
+				zoomFrom: '1915-08-01',
+				zoomTo: '1915-08-31'
+			})
+		});
+
+		const resetBtn = document.querySelector(
+			'[data-testid="timeline-zoom-reset"]'
+		) as HTMLButtonElement;
+		resetBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(goto).toHaveBeenCalledOnce();
+		const [url] = vi.mocked(goto).mock.calls[0];
+		expect(url).not.toContain('zoomFrom=');
+		expect(url).not.toContain('zoomTo=');
+	});
 });