diff --git a/src/components/ai-edition/NewEditorShell.timelineHeight.test.tsx b/src/components/ai-edition/NewEditorShell.timelineHeight.test.tsx
new file mode 100644
index 000000000..d16ce4492
--- /dev/null
+++ b/src/components/ai-edition/NewEditorShell.timelineHeight.test.tsx
@@ -0,0 +1,235 @@
+// @vitest-environment jsdom
+import "@testing-library/jest-dom";
+import { act, cleanup, fireEvent, render } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/contexts/ShortcutsContext", async () => {
+	const { DEFAULT_SHORTCUTS } = await import("@/lib/shortcuts");
+	return {
+		useShortcuts: () => ({
+			shortcuts: DEFAULT_SHORTCUTS,
+			isMac: false,
+			isConfigOpen: false,
+			openConfig: vi.fn(),
+			closeConfig: vi.fn(),
+			setShortcuts: vi.fn(),
+			persistShortcuts: () => Promise.resolve(true),
+		}),
+	};
+});
+
+vi.mock("@/contexts/I18nContext", () => ({
+	useI18n: () => ({
+		locale: "en",
+		setLocale: vi.fn(),
+	}),
+	useScopedT: () => (key: string) => key,
+}));
+
+import { EditorDialogsProvider } from "@/contexts/EditorDialogsContext";
+import { AUDIO_ROW_EXPANSION_PX } from "@/lib/ai-edition/document/audioTracks";
+import { createAudioTrack, createEmptyDocument } from "@/lib/ai-edition/schema";
+import { useProjectStore } from "@/lib/ai-edition/store/projectStore";
+import {
+	DEFAULT_TIMELINE_HEIGHT_PX,
+	MAX_TIMELINE_HEIGHT_PX,
+	MIN_TIMELINE_HEIGHT_PX,
+	NewEditorShell,
+} from "./NewEditorShell";
+
+function renderShell() {
+	return render(
+		<EditorDialogsProvider>
+			<NewEditorShell />
+		</EditorDialogsProvider>,
+	);
+}
+
+describe("NewEditorShell timeline height", () => {
+	beforeEach(() => {
+		localStorage.clear();
+		(window as unknown as { electronAPI?: unknown }).electronAPI = {
+			onAiEditionChatEvent: () => () => {
+				/* unsubscribe */
+			},
+			sendAiEditionChatPrompt: () => {
+				/* mock */
+			},
+			setTitleBarOverlay: () => {
+				/* noop */
+			},
+			setHasUnsavedChanges: () => {
+				/* noop */
+			},
+			onRequestCloseConfirm: () => () => {
+				/* unsubscribe */
+			},
+			onRequestSaveBeforeClose: () => () => {
+				/* unsubscribe */
+			},
+			sendCloseConfirmResponse: () => {
+				/* noop */
+			},
+			findRecordingCamera: () => Promise.resolve(null),
+			preparePreviewAudioTrack: () => Promise.resolve(null),
+			isAppPackaged: () => false,
+		};
+		Element.prototype.scrollTo = () => {
+			/* no scrolling in jsdom */
+		};
+		(globalThis as unknown as { ResizeObserver?: unknown }).ResizeObserver = class {
+			observe() {
+				/* noop */
+			}
+			unobserve() {
+				/* noop */
+			}
+			disconnect() {
+				/* noop */
+			}
+		};
+	});
+
+	afterEach(() => {
+		cleanup();
+		localStorage.clear();
+	});
+
+	it("exports expected constants allowing all lanes to fit without vertical clipping", () => {
+		expect(DEFAULT_TIMELINE_HEIGHT_PX).toBe(392);
+		expect(MIN_TIMELINE_HEIGHT_PX).toBe(160);
+		expect(MAX_TIMELINE_HEIGHT_PX).toBe(560);
+	});
+
+	it("uses DEFAULT_TIMELINE_HEIGHT_PX on first opening when localStorage has no saved height", () => {
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root).not.toBeNull();
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${DEFAULT_TIMELINE_HEIGHT_PX}px`);
+	});
+
+	it("migrates legacy cramped 308px default to DEFAULT_TIMELINE_HEIGHT_PX and persists it", () => {
+		localStorage.setItem("os-editor-timeline-height", "308");
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${DEFAULT_TIMELINE_HEIGHT_PX}px`);
+		expect(localStorage.getItem("os-editor-timeline-height")).toBe(
+			String(DEFAULT_TIMELINE_HEIGHT_PX),
+		);
+		expect(localStorage.getItem("os-editor-timeline-height-migrated")).toBe("true");
+	});
+
+	it("migrates intermediate 344px default to DEFAULT_TIMELINE_HEIGHT_PX and persists it", () => {
+		localStorage.setItem("os-editor-timeline-height", "344");
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${DEFAULT_TIMELINE_HEIGHT_PX}px`);
+		expect(localStorage.getItem("os-editor-timeline-height")).toBe(
+			String(DEFAULT_TIMELINE_HEIGHT_PX),
+		);
+	});
+
+	it("preserves an intentional user choice of 308px after migration has run", () => {
+		localStorage.setItem("os-editor-timeline-height-migrated", "true");
+		localStorage.setItem("os-editor-timeline-height", "308");
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root.style.gridTemplateRows).toBe("58px 1fr 308px");
+	});
+
+	it("respects a custom user preference saved in localStorage within valid bounds", () => {
+		localStorage.setItem("os-editor-timeline-height", "450");
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root.style.gridTemplateRows).toBe("58px 1fr 450px");
+	});
+
+	it("clamps out-of-bounds custom heights from localStorage at lower and upper bounds", () => {
+		localStorage.setItem("os-editor-timeline-height-migrated", "true");
+		localStorage.setItem("os-editor-timeline-height", "50");
+		const { container: c1 } = renderShell();
+		const root1 = c1.firstElementChild as HTMLElement;
+		expect(root1.style.gridTemplateRows).toBe(`58px 1fr ${MIN_TIMELINE_HEIGHT_PX}px`);
+
+		cleanup();
+		localStorage.setItem("os-editor-timeline-height", "999");
+		const { container: c2 } = renderShell();
+		const root2 = c2.firstElementChild as HTMLElement;
+		expect(root2.style.gridTemplateRows).toBe(`58px 1fr ${MAX_TIMELINE_HEIGHT_PX}px`);
+	});
+
+	it("clamps pointer resizing through startTimelineResize to MIN and MAX bounds", () => {
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		const handle = container.querySelector(
+			'[role="separator"][aria-orientation="horizontal"]',
+		) as HTMLElement;
+		expect(handle).not.toBeNull();
+
+		// Drag down significantly (clientY increases): should clamp to MIN_TIMELINE_HEIGHT_PX
+		act(() => {
+			fireEvent.pointerDown(handle, { clientY: 400 });
+			fireEvent.pointerMove(window, { clientY: 1000 });
+		});
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${MIN_TIMELINE_HEIGHT_PX}px`);
+		act(() => {
+			fireEvent.pointerUp(window);
+		});
+		expect(localStorage.getItem("os-editor-timeline-height")).toBe(String(MIN_TIMELINE_HEIGHT_PX));
+
+		// Drag up significantly (clientY decreases): should clamp to MAX_TIMELINE_HEIGHT_PX
+		act(() => {
+			fireEvent.pointerDown(handle, { clientY: 400 });
+			fireEvent.pointerMove(window, { clientY: -500 });
+		});
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${MAX_TIMELINE_HEIGHT_PX}px`);
+		act(() => {
+			fireEvent.pointerUp(window);
+		});
+		expect(localStorage.getItem("os-editor-timeline-height")).toBe(String(MAX_TIMELINE_HEIGHT_PX));
+	});
+
+	it("dynamically expands height by AUDIO_ROW_EXPANSION_PX when transitioning from 1 to 2 audio lanes and shrinks back", () => {
+		const { container } = renderShell();
+		const root = container.firstElementChild as HTMLElement;
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${DEFAULT_TIMELINE_HEIGHT_PX}px`);
+
+		// Add voiceover and music tracks (creating 2 audio rows)
+		const doc = createEmptyDocument({ projectId: "p", title: "t" });
+		doc.audioTracks = [
+			createAudioTrack({
+				assetId: "a1",
+				durationSec: 5,
+				timelineStartSec: 0,
+				spanSec: 5,
+				kind: "voiceover",
+			}),
+			createAudioTrack({
+				assetId: "a2",
+				durationSec: 5,
+				timelineStartSec: 0,
+				spanSec: 5,
+				kind: "music",
+			}),
+		];
+
+		act(() => {
+			useProjectStore.setState({ document: doc });
+		});
+
+		const expectedExpandedHeight = DEFAULT_TIMELINE_HEIGHT_PX + AUDIO_ROW_EXPANSION_PX;
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${expectedExpandedHeight}px`);
+
+		// Remove music track (returning to 1 audio row)
+		const singleTrackDoc = {
+			...doc,
+			audioTracks: [doc.audioTracks[0]],
+		};
+
+		act(() => {
+			useProjectStore.setState({ document: singleTrackDoc });
+		});
+
+		expect(root.style.gridTemplateRows).toBe(`58px 1fr ${DEFAULT_TIMELINE_HEIGHT_PX}px`);
+	});
+});
diff --git a/src/components/ai-edition/NewEditorShell.tsx b/src/components/ai-edition/NewEditorShell.tsx
index 6404c3fdb..1a5be2ab5 100644
--- a/src/components/ai-edition/NewEditorShell.tsx
+++ b/src/components/ai-edition/NewEditorShell.tsx
@@ -5,6 +5,10 @@ import { toFileUrl } from "@/components/video-editor/projectPersistence";
 import { useEditorDialogActions } from "@/contexts/EditorDialogsContext";
 import { useScopedT } from "@/contexts/I18nContext";
 import { useShortcuts } from "@/contexts/ShortcutsContext";
+import {
+	AUDIO_ROW_EXPANSION_PX,
+	computeAudioRowCount,
+} from "@/lib/ai-edition/document/audioTracks";
 import { createId } from "@/lib/ai-edition/document/ids";
 import {
 	migrateProjectDataToAxcutDocument,
@@ -106,6 +110,10 @@ function NativePlaybackSync({
 	return null;
 }
 
+export const DEFAULT_TIMELINE_HEIGHT_PX = 392;
+export const MIN_TIMELINE_HEIGHT_PX = 160;
+export const MAX_TIMELINE_HEIGHT_PX = 560;
+
 export function NewEditorShell() {
 	const te = useScopedT("editor");
 	const document = useProjectStore((s) => s.document);
@@ -132,8 +140,39 @@ export function NewEditorShell() {
 	const [chatWidthPx, setChatWidthPx] = useState(
 		() => Number(localStorage.getItem("os-editor-chat-width")) || 392,
 	);
-	const [timelineHeightPx, setTimelineHeightPx] = useState(
-		() => Number(localStorage.getItem("os-editor-timeline-height")) || 308,
+	const [timelineBaseHeightPx, setTimelineBaseHeightPx] = useState(() => {
+		const raw = localStorage.getItem("os-editor-timeline-height");
+		// One-time migration: if legacy 308px default exists without the migration marker,
+		// migrate it once to DEFAULT_TIMELINE_HEIGHT_PX so existing users see all lanes,
+		// but allow them to intentionally choose 308px in the future.
+		// Also migrate intermediate 344px default to DEFAULT_TIMELINE_HEIGHT_PX.
+		const migrated = localStorage.getItem("os-editor-timeline-height-migrated");
+		if (!migrated && (!raw || Number(raw) === 308 || Number(raw) === 344)) {
+			localStorage.setItem("os-editor-timeline-height-migrated", "true");
+			localStorage.setItem("os-editor-timeline-height", String(DEFAULT_TIMELINE_HEIGHT_PX));
+			return DEFAULT_TIMELINE_HEIGHT_PX;
+		}
+		if (raw === "344") {
+			localStorage.setItem("os-editor-timeline-height", String(DEFAULT_TIMELINE_HEIGHT_PX));
+			return DEFAULT_TIMELINE_HEIGHT_PX;
+		}
+		const val = raw ? Number(raw) : 0;
+		if (!val) {
+			return DEFAULT_TIMELINE_HEIGHT_PX;
+		}
+		return Math.min(MAX_TIMELINE_HEIGHT_PX, Math.max(MIN_TIMELINE_HEIGHT_PX, val));
+	});
+
+	// The timeline height automatically expands by AUDIO_ROW_EXPANSION_PX (+29px)
+	// whenever the project transitions between 1 and 2 stacked audio rows (e.g. voiceover + music).
+	const audioRowCount = useMemo(
+		() => computeAudioRowCount(document?.audioTracks ?? []),
+		[document?.audioTracks],
+	);
+	const extraAudioHeightPx = Math.max(0, audioRowCount - 1) * AUDIO_ROW_EXPANSION_PX;
+	const timelineHeightPx = Math.min(
+		MAX_TIMELINE_HEIGHT_PX,
+		Math.max(MIN_TIMELINE_HEIGHT_PX, timelineBaseHeightPx + extraAudioHeightPx),
 	);
 	const [inspectorOpen, setInspectorOpen] = useState(true);
 	const [facet, setFacet] = useState<Facet>("effects");
@@ -1391,23 +1430,27 @@ export function NewEditorShell() {
 		(e: React.PointerEvent) => {
 			e.preventDefault();
 			const startY = e.clientY;
-			const startHeight = timelineHeightPx;
-			let latest = startHeight;
+			const startBase = timelineBaseHeightPx;
+			let latestBase = startBase;
 			const move = (ev: PointerEvent) => {
 				// Dragging the handle up (negative clientY delta) enlarges the
 				// timeline, since it sits below the handle.
-				latest = Math.min(480, Math.max(160, startHeight - (ev.clientY - startY)));
-				setTimelineHeightPx(latest);
+				const renderedTarget = Math.min(
+					MAX_TIMELINE_HEIGHT_PX,
+					Math.max(MIN_TIMELINE_HEIGHT_PX, startBase + extraAudioHeightPx - (ev.clientY - startY)),
+				);
+				latestBase = Math.max(MIN_TIMELINE_HEIGHT_PX, renderedTarget - extraAudioHeightPx);
+				setTimelineBaseHeightPx(latestBase);
 			};
 			const up = () => {
 				window.removeEventListener("pointermove", move);
 				window.removeEventListener("pointerup", up);
-				localStorage.setItem("os-editor-timeline-height", String(latest));
+				localStorage.setItem("os-editor-timeline-height", String(latestBase));
 			};
 			window.addEventListener("pointermove", move);
 			window.addEventListener("pointerup", up);
 		},
-		[timelineHeightPx],
+		[timelineBaseHeightPx, extraAudioHeightPx],
 	);
 
 	const transcriptProps = {
diff --git a/src/components/ai-edition/v4/V4Timeline.tsx b/src/components/ai-edition/v4/V4Timeline.tsx
index 3fd5a82ce..a4b41ff32 100644
--- a/src/components/ai-edition/v4/V4Timeline.tsx
+++ b/src/components/ai-edition/v4/V4Timeline.tsx
@@ -34,6 +34,9 @@ import { useScopedT } from "@/contexts/I18nContext";
 import { useShortcuts } from "@/contexts/ShortcutsContext";
 import { useAudioPeaks } from "@/hooks/useAudioPeaks";
 import {
+	AUDIO_LANE_PAD_PX,
+	AUDIO_ROW_GAP_PX,
+	AUDIO_ROW_HEIGHT_PX,
 	audioGhostExtent,
 	collapseTracksToPills,
 	packAudioTrackRows,
@@ -141,14 +144,8 @@ const PILL_HANDLE_OUT_PX = PILL_HANDLE_PX + PILL_MOVE_GAP_PX;
 const PILL_CONTENT_MIN_PX = 34;
 /** Edge-snap radius while dragging a pill, in screen px. */
 const PILL_SNAP_PX = 8;
-// One audio pill's height, and the vertical step between stacked rows. The lane
-// grows by a row for each track that overlaps one already placed — see
-// `packAudioTrackRows`.
-const AUDIO_ROW_HEIGHT_PX = 26;
-const AUDIO_ROW_GAP_PX = 3;
-// Breathing room above the first row and below the last, so a pill never sits
-// flush against the lane's rounded edge.
-const AUDIO_LANE_PAD_PX = 3;
+// One audio pill's height, the vertical step between stacked rows, and lane padding
+// are defined in audioTracks.ts and imported above.
 // The size a newly created pill aims for (PILL_CREATE_PX) lives in
 // timeline/newRegionDuration, because the keyboard shortcuts create regions too
 // and they are handled in NewEditorShell, outside this component.
diff --git a/src/lib/ai-edition/document/audioTracks.test.ts b/src/lib/ai-edition/document/audioTracks.test.ts
index 064771150..25702630a 100644
--- a/src/lib/ai-edition/document/audioTracks.test.ts
+++ b/src/lib/ai-edition/document/audioTracks.test.ts
@@ -4,6 +4,7 @@ import {
 	anchorAudioTrackFragments,
 	audioGhostExtent,
 	collapseTracksToPills,
+	computeAudioRowCount,
 	packAudioTrackRows,
 	patchAudioTrack,
 	removeAudioTrack,
@@ -356,3 +357,37 @@ describe("slipAudioOffsetMs", () => {
 		expect(Number.isInteger(slipAudioOffsetMs(0, 4_000, 60, 1234.567) ?? 0)).toBe(true);
 	});
 });
+
+describe("computeAudioRowCount", () => {
+	it("returns 1 row for an empty tracks list (showing the empty lane hint)", () => {
+		expect(computeAudioRowCount([])).toBe(1);
+	});
+
+	it("returns 1 row for a single voiceover track", () => {
+		const vo = track({ id: "vo1", kind: "voiceover", startMs: 0, endMs: 5000 });
+		expect(computeAudioRowCount([vo])).toBe(1);
+	});
+
+	it("returns 1 row for a single music track", () => {
+		const bgm = track({ id: "bgm1", kind: "music", startMs: 0, endMs: 5000 });
+		expect(computeAudioRowCount([bgm])).toBe(1);
+	});
+
+	it("returns 2 rows when both voiceover and music tracks are present", () => {
+		const vo = track({ id: "vo1", kind: "voiceover", startMs: 0, endMs: 5000 });
+		const bgm = track({ id: "bgm1", kind: "music", startMs: 0, endMs: 5000 });
+		expect(computeAudioRowCount([vo, bgm])).toBe(2);
+	});
+
+	it("keeps non-overlapping tracks of the same kind on 1 row", () => {
+		const bgm1 = track({ id: "bgm1", kind: "music", startMs: 0, endMs: 5000 });
+		const bgm2 = track({ id: "bgm2", kind: "music", startMs: 6000, endMs: 10000 });
+		expect(computeAudioRowCount([bgm1, bgm2])).toBe(1);
+	});
+
+	it("stacks overlapping tracks of the same kind on 2 rows", () => {
+		const bgm1 = track({ id: "bgm1", kind: "music", startMs: 0, endMs: 5000 });
+		const bgm2 = track({ id: "bgm2", kind: "music", startMs: 3000, endMs: 8000 });
+		expect(computeAudioRowCount([bgm1, bgm2])).toBe(2);
+	});
+});
diff --git a/src/lib/ai-edition/document/audioTracks.ts b/src/lib/ai-edition/document/audioTracks.ts
index 232d72af8..b0055cb3a 100644
--- a/src/lib/ai-edition/document/audioTracks.ts
+++ b/src/lib/ai-edition/document/audioTracks.ts
@@ -500,3 +500,22 @@ export function packAudioTrackRows(tracks: Array<{ id: string; startMs: number;
 	}
 	return { rowOf, rowCount: Math.max(1, rowEnds.length) };
 }
+
+export const AUDIO_ROW_HEIGHT_PX = 26;
+export const AUDIO_ROW_GAP_PX = 3;
+export const AUDIO_LANE_PAD_PX = 3;
+export const AUDIO_ROW_EXPANSION_PX = AUDIO_ROW_HEIGHT_PX + AUDIO_ROW_GAP_PX;
+
+/**
+ * Compute the total stacked rows required to render the project's audio tracks,
+ * mirroring V4Timeline's packing logic (separating voiceovers and background music).
+ */
+export function computeAudioRowCount(tracks: AxcutAudioTrack[]): number {
+	const pills = collapseTracksToPills(tracks);
+	const voice = pills.filter((p) => p.kind === "voiceover");
+	const music = pills.filter((p) => p.kind !== "voiceover");
+	const voiceRows = packAudioTrackRows(voice);
+	const musicRows = packAudioTrackRows(music);
+	const base = voice.length > 0 ? voiceRows.rowCount : 0;
+	return Math.max(1, base + (music.length > 0 ? musicRows.rowCount : 0));
+}