diff --git a/frontend/src/lib/document/TimelineDensityFilter.svelte b/frontend/src/lib/document/TimelineDensityFilter.svelte
index 5c8e99b6..5f7b8946 100644
--- a/frontend/src/lib/document/TimelineDensityFilter.svelte
+++ b/frontend/src/lib/document/TimelineDensityFilter.svelte
@@ -8,6 +8,7 @@ import {
 	selectionBoundaryTo,
 	formatTickLabel
 } from '$lib/document/timeline';
+import { createTimelineDrag } from '$lib/document/useTimelineDrag.svelte';
 import { getLocale } from '$lib/paraglide/runtime';
 import TimelineBars from '$lib/document/TimelineBars.svelte';
 import TimelineYAxis from '$lib/document/TimelineYAxis.svelte';
@@ -73,25 +74,7 @@ const maxCount = $derived(Math.max(...filled.map((b) => b.count), 1));
 
 const hasSelection = $derived(from !== '' || to !== '');
 
-let dragStartIndex: number | null = $state(null);
-let dragEndIndex: number | null = $state(null);
 let rowEl: HTMLDivElement | undefined = $state();
-// Set when a pointerup just emitted a selection so the synthesized click that
-// follows mouse interaction is suppressed (we'd otherwise emit twice). Keyboard
-// Enter/Space on a focused button still fires `click` without preceding
-// pointerdown, so the click path remains the keyboard accessibility surface.
-let suppressClick = $state(false);
-const isDragging = $derived(dragStartIndex !== null);
-const dragLowIndex = $derived(
-	dragStartIndex !== null && dragEndIndex !== null
-		? Math.min(dragStartIndex, dragEndIndex)
-		: dragStartIndex
-);
-const dragHighIndex = $derived(
-	dragStartIndex !== null && dragEndIndex !== null
-		? Math.max(dragStartIndex, dragEndIndex)
-		: dragStartIndex
-);
 
 function clearSelection() {
 	onchange({ from: '', to: '' });
@@ -103,10 +86,8 @@ function isSelected(label: string): boolean {
 	return labelFrom >= from && labelFrom <= to;
 }
 
-function isInDragPreview(index: number): boolean {
-	if (!isDragging) return false;
-	if (dragLowIndex === null || dragHighIndex === null) return false;
-	return index >= dragLowIndex && index <= dragHighIndex;
+function isYearLabel(label: string): boolean {
+	return label.length === 4;
 }
 
 function emitSelection(startIndex: number, endIndex: number, includeZoom: boolean) {
@@ -138,102 +119,39 @@ function indexFromClientX(clientX: number): number | null {
 	return Math.min(filled.length - 1, Math.max(0, Math.floor(x / barWidth)));
 }
 
-function handleDocumentMove(e: PointerEvent) {
-	const idx = indexFromClientX(e.clientX);
-	if (idx !== null) dragEndIndex = idx;
-}
-
-function handleDocumentUp() {
-	cleanupDragListeners();
-	finalizeDrag();
-}
-
-function handleDocumentCancel() {
-	cleanupDragListeners();
-	dragStartIndex = null;
-	dragEndIndex = null;
-}
-
-function cleanupDragListeners() {
-	document.removeEventListener('pointermove', handleDocumentMove);
-	document.removeEventListener('pointerup', handleDocumentUp);
-	document.removeEventListener('pointercancel', handleDocumentCancel);
-}
+const drag = createTimelineDrag({
+	indexFromClientX,
+	labelAt: (i) => filled[i]?.month,
+	isYearLabel,
+	emit: emitSelection
+});
 
 // Strip any in-flight document listeners if the component unmounts mid-drag
 // (route change, view toggle, breakpoint drop). Without this they survive on
 // document and keep writing to torn-down state cells.
-$effect(() => {
-	return cleanupDragListeners;
-});
+$effect(() => drag.cleanup);
 
-function finalizeDrag() {
-	if (dragStartIndex === null || dragEndIndex === null) return;
-	const start = dragStartIndex;
-	const end = dragEndIndex;
-	dragStartIndex = null;
-	dragEndIndex = null;
-	const isRangeDrag = start !== end;
-	const startLabel = filled[start]?.month;
-	// Range drag → atomic zoom + filter.
-	// Same-bar release on a year bar → zoom into that year's months.
-	// Same-bar release on a month bar → filter only.
-	const includeZoom = isRangeDrag || (!!startLabel && isYearLabel(startLabel));
-	emitSelection(start, end, includeZoom);
-	// Suppress the synthesized click that follows pointerup so we don't
-	// double-emit. Keyboard Enter/Space fires click without a preceding
-	// pointerup and stays the keyboard accessibility surface.
-	suppressClick = true;
-	queueMicrotask(() => {
-		suppressClick = false;
-	});
-}
-
-function handlePointerDown(event: PointerEvent, index: number) {
-	if (event.button !== 0) return;
-	dragStartIndex = index;
-	dragEndIndex = index;
-	document.addEventListener('pointermove', handleDocumentMove);
-	document.addEventListener('pointerup', handleDocumentUp);
-	document.addEventListener('pointercancel', handleDocumentCancel);
-}
-
-function handlePointerEnter(index: number) {
-	if (!isDragging) return;
-	dragEndIndex = index;
-}
-
-function isYearLabel(label: string): boolean {
-	return label.length === 4;
-}
-
-function handleClick(index: number) {
-	if (suppressClick) {
-		suppressClick = false;
-		return;
-	}
-	const label = filled[index]?.month;
-	// Click on a year bar zooms into that year so the user can see month-level
-	// density. Click on a month bar just filters.
-	const includeZoom = !!label && isYearLabel(label);
-	emitSelection(index, index, includeZoom);
+function isInDragPreview(index: number): boolean {
+	if (!drag.isDragging) return false;
+	if (drag.lowIndex === null || drag.highIndex === null) return false;
+	return index >= drag.lowIndex && index <= drag.highIndex;
 }
 
 const dragWindowLeftPct = $derived.by(() => {
-	if (!isDragging || dragLowIndex === null || filled.length === 0) return 0;
-	return (dragLowIndex / filled.length) * 100;
+	if (!drag.isDragging || drag.lowIndex === null || filled.length === 0) return 0;
+	return (drag.lowIndex / filled.length) * 100;
 });
 const dragWindowRightPct = $derived.by(() => {
-	if (!isDragging || dragHighIndex === null || filled.length === 0) return 100;
-	return ((filled.length - dragHighIndex - 1) / filled.length) * 100;
+	if (!drag.isDragging || drag.highIndex === null || filled.length === 0) return 100;
+	return ((filled.length - drag.highIndex - 1) / filled.length) * 100;
 });
 
 // While dragging, expose the live preview range to assistive tech via a
 // polite live region. Empty text outside drag avoids announcing residual state.
 const dragLiveMessage = $derived.by(() => {
-	if (!isDragging || dragLowIndex === null || dragHighIndex === null) return '';
-	const fromLabel = filled[dragLowIndex]?.month;
-	const toLabel = filled[dragHighIndex]?.month;
+	if (!drag.isDragging || drag.lowIndex === null || drag.highIndex === null) return '';
+	const fromLabel = filled[drag.lowIndex]?.month;
+	const toLabel = filled[drag.highIndex]?.month;
 	if (!fromLabel || !toLabel) return '';
 	return m.timeline_dragging_aria_live({
 		from: formatTickLabel(fromLabel, getLocale()),
@@ -259,13 +177,13 @@ const dragLiveMessage = $derived.by(() => {
 					barAreaHeight={BAR_AREA_HEIGHT}
 					isSelected={isSelected}
 					isInDragPreview={isInDragPreview}
-					isDragging={isDragging}
+					isDragging={drag.isDragging}
 					dragWindowLeftPct={dragWindowLeftPct}
 					dragWindowRightPct={dragWindowRightPct}
 					bind:rowEl={rowEl}
-					onbarpointerdown={handlePointerDown}
-					onbarpointerenter={handlePointerEnter}
-					onbarclick={handleClick}
+					onbarpointerdown={drag.pointerDown}
+					onbarpointerenter={drag.pointerEnter}
+					onbarclick={drag.click}
 				/>
 
 				<TimelineXAxis filled={filled} />
diff --git a/frontend/src/lib/document/useTimelineDrag.svelte.test.ts b/frontend/src/lib/document/useTimelineDrag.svelte.test.ts
new file mode 100644
index 00000000..2bf473b0
--- /dev/null
+++ b/frontend/src/lib/document/useTimelineDrag.svelte.test.ts
@@ -0,0 +1,155 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { flushSync } from 'svelte';
+
+const { createTimelineDrag } = await import('./useTimelineDrag.svelte');
+
+type EmitCall = { start: number; end: number; includeZoom: boolean };
+
+function makeOpts(overrides: Partial<Parameters<typeof createTimelineDrag>[0]> = {}) {
+	const labels = ['1915-08', '1915-09', '1915-10', '1915']; // last entry is a year label
+	const calls: EmitCall[] = [];
+	const opts = {
+		indexFromClientX: vi.fn(() => 0),
+		labelAt: (i: number) => labels[i],
+		isYearLabel: (l: string) => l.length === 4,
+		emit: (start: number, end: number, includeZoom: boolean) => {
+			calls.push({ start, end, includeZoom });
+		},
+		...overrides
+	};
+	return { opts, calls };
+}
+
+function pointerDownEvent(button = 0): PointerEvent {
+	return { button } as unknown as PointerEvent;
+}
+
+describe('createTimelineDrag', () => {
+	beforeEach(() => {
+		// Reset listeners attached to the JSDOM document between tests
+		document.removeEventListener('pointermove', () => undefined);
+	});
+	afterEach(() => {
+		vi.restoreAllMocks();
+	});
+
+	it('starts with no drag in progress and null low/high indices', () => {
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		expect(drag.isDragging).toBe(false);
+		expect(drag.lowIndex).toBeNull();
+		expect(drag.highIndex).toBeNull();
+	});
+
+	it('pointerDown on a primary button enters drag state at that index', () => {
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 1);
+		flushSync();
+		expect(drag.isDragging).toBe(true);
+		expect(drag.lowIndex).toBe(1);
+		expect(drag.highIndex).toBe(1);
+	});
+
+	it('pointerDown on a non-primary button is ignored', () => {
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(2), 1);
+		flushSync();
+		expect(drag.isDragging).toBe(false);
+	});
+
+	it('pointerEnter during drag widens the range high boundary', () => {
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 0);
+		drag.pointerEnter(2);
+		flushSync();
+		expect(drag.lowIndex).toBe(0);
+		expect(drag.highIndex).toBe(2);
+	});
+
+	it('pointerEnter outside drag is a no-op', () => {
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerEnter(2);
+		flushSync();
+		expect(drag.isDragging).toBe(false);
+		expect(drag.lowIndex).toBeNull();
+	});
+
+	it('click on a month bar emits filter only (no zoom)', () => {
+		const { opts, calls } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.click(0);
+		expect(calls).toEqual([{ start: 0, end: 0, includeZoom: false }]);
+	});
+
+	it('click on a year bar emits filter + zoom atomically', () => {
+		const { opts, calls } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.click(3); // labels[3] = '1915' (year label)
+		expect(calls).toEqual([{ start: 3, end: 3, includeZoom: true }]);
+	});
+
+	it('range drag commits emit with zoom in ascending order', async () => {
+		const { opts, calls } = makeOpts({ indexFromClientX: vi.fn(() => 2) });
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 0);
+		document.dispatchEvent(new PointerEvent('pointermove', { clientX: 999 }));
+		document.dispatchEvent(new PointerEvent('pointerup'));
+		flushSync();
+		expect(calls).toEqual([{ start: 0, end: 2, includeZoom: true }]);
+		expect(drag.isDragging).toBe(false);
+	});
+
+	it('reverse-direction drag still emits ascending boundaries via emit ordering', async () => {
+		const { opts, calls } = makeOpts({ indexFromClientX: vi.fn(() => 0) });
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 2);
+		document.dispatchEvent(new PointerEvent('pointermove', { clientX: 0 }));
+		document.dispatchEvent(new PointerEvent('pointerup'));
+		flushSync();
+		// emit receives raw start/end — orchestrator's emit() handles ordering
+		expect(calls).toEqual([{ start: 2, end: 0, includeZoom: true }]);
+	});
+
+	it('pointerup on the same bar (no-range) on a month label emits filter without zoom', () => {
+		const { opts, calls } = makeOpts({ indexFromClientX: vi.fn(() => 0) });
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 0);
+		document.dispatchEvent(new PointerEvent('pointerup'));
+		flushSync();
+		expect(calls).toEqual([{ start: 0, end: 0, includeZoom: false }]);
+	});
+
+	it('pointercancel resets state without emitting', () => {
+		const { opts, calls } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 1);
+		document.dispatchEvent(new PointerEvent('pointercancel'));
+		flushSync();
+		expect(calls).toEqual([]);
+		expect(drag.isDragging).toBe(false);
+	});
+
+	it('click after pointerup is suppressed (no double emit)', async () => {
+		const { opts, calls } = makeOpts({ indexFromClientX: vi.fn(() => 0) });
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 0);
+		document.dispatchEvent(new PointerEvent('pointerup'));
+		flushSync();
+		drag.click(0); // synthesized click after pointerup
+		expect(calls.length).toBe(1); // only the pointerup-driven emit, not the click
+	});
+
+	it('cleanup removes document pointer listeners', () => {
+		const removeSpy = vi.spyOn(document, 'removeEventListener');
+		const { opts } = makeOpts();
+		const drag = createTimelineDrag(opts);
+		drag.pointerDown(pointerDownEvent(0), 0);
+		drag.cleanup();
+		const removed = removeSpy.mock.calls.map((c) => c[0]);
+		expect(removed).toEqual(expect.arrayContaining(['pointermove', 'pointerup', 'pointercancel']));
+	});
+});
diff --git a/frontend/src/lib/document/useTimelineDrag.svelte.ts b/frontend/src/lib/document/useTimelineDrag.svelte.ts
new file mode 100644
index 00000000..f35d623e
--- /dev/null
+++ b/frontend/src/lib/document/useTimelineDrag.svelte.ts
@@ -0,0 +1,118 @@
+type DragOptions = {
+	/**
+	 * Maps a viewport X coordinate to a bar index, or null if outside the row.
+	 * The orchestrator owns the row element bound from `TimelineBars`.
+	 */
+	indexFromClientX(clientX: number): number | null;
+	/** Returns the bucket label at index, or undefined if out of range. */
+	labelAt(index: number): string | undefined;
+	/**
+	 * True when a label represents an aggregated year bar — controls click-to-zoom
+	 * semantics (clicking a year zooms into its 12 months; clicking a month doesn't).
+	 */
+	isYearLabel(label: string): boolean;
+	/**
+	 * Emits a selection. `includeZoom` is true for a range drag or a click on a
+	 * year bar; false for a click on a month bar.
+	 */
+	emit(startIndex: number, endIndex: number, includeZoom: boolean): void;
+};
+
+/**
+ * Drag state machine for the timeline density widget. Exposes:
+ * - `isDragging`, `lowIndex`, `highIndex` reactive read-onlies for the UI
+ * - `pointerDown` / `pointerEnter` / `click` handlers for bar buttons
+ * - `cleanup()` to drop document-level listeners on component unmount
+ *
+ * Document-level listeners (pointermove, pointerup, pointercancel) keep drag
+ * tracking alive while the cursor leaves the original bar or the timeline row.
+ * `cleanup()` must be called from a Svelte `$effect` teardown so a route change
+ * mid-drag does not leak listeners.
+ */
+export function createTimelineDrag(opts: DragOptions) {
+	let startIndex = $state<number | null>(null);
+	let endIndex = $state<number | null>(null);
+	// Set after a pointerup-driven emit so the synthesized click that follows is
+	// suppressed (we'd otherwise emit twice). Keyboard Enter/Space fires click
+	// without preceding pointerdown, so click stays the keyboard surface.
+	let suppressClick = $state(false);
+
+	function handleDocumentMove(e: PointerEvent) {
+		const idx = opts.indexFromClientX(e.clientX);
+		if (idx !== null) endIndex = idx;
+	}
+
+	function handleDocumentUp() {
+		cleanup();
+		finalizeDrag();
+	}
+
+	function handleDocumentCancel() {
+		cleanup();
+		startIndex = null;
+		endIndex = null;
+	}
+
+	function cleanup() {
+		document.removeEventListener('pointermove', handleDocumentMove);
+		document.removeEventListener('pointerup', handleDocumentUp);
+		document.removeEventListener('pointercancel', handleDocumentCancel);
+	}
+
+	function finalizeDrag() {
+		if (startIndex === null || endIndex === null) return;
+		const start = startIndex;
+		const end = endIndex;
+		startIndex = null;
+		endIndex = null;
+		const isRangeDrag = start !== end;
+		const startLabel = opts.labelAt(start);
+		// Range drag → atomic zoom + filter. Same-bar release on a year bar →
+		// zoom into that year's months. Same-bar release on a month bar →
+		// filter only.
+		const includeZoom = isRangeDrag || (!!startLabel && opts.isYearLabel(startLabel));
+		opts.emit(start, end, includeZoom);
+		suppressClick = true;
+		queueMicrotask(() => {
+			suppressClick = false;
+		});
+	}
+
+	return {
+		get isDragging() {
+			return startIndex !== null;
+		},
+		get lowIndex() {
+			if (startIndex === null) return null;
+			if (endIndex === null) return startIndex;
+			return Math.min(startIndex, endIndex);
+		},
+		get highIndex() {
+			if (startIndex === null) return null;
+			if (endIndex === null) return startIndex;
+			return Math.max(startIndex, endIndex);
+		},
+		pointerDown(event: PointerEvent, index: number) {
+			if (event.button !== 0) return;
+			startIndex = index;
+			endIndex = index;
+			document.addEventListener('pointermove', handleDocumentMove);
+			document.addEventListener('pointerup', handleDocumentUp);
+			document.addEventListener('pointercancel', handleDocumentCancel);
+		},
+		pointerEnter(index: number) {
+			if (startIndex === null) return;
+			endIndex = index;
+		},
+		click(index: number) {
+			if (suppressClick) {
+				suppressClick = false;
+				return;
+			}
+			const label = opts.labelAt(index);
+			const includeZoom = !!label && opts.isYearLabel(label);
+			opts.emit(index, index, includeZoom);
+		},
+		cleanup
+	};
+}