diff --git a/packages/studio/src/player/lib/timelinePerformanceDiagnostics.test.ts b/packages/studio/src/player/lib/timelinePerformanceDiagnostics.test.ts
new file mode 100644
index 0000000000..8e2f44741b
--- /dev/null
+++ b/packages/studio/src/player/lib/timelinePerformanceDiagnostics.test.ts
@@ -0,0 +1,85 @@
+// @vitest-environment happy-dom
+import { afterEach, describe, expect, it } from "vitest";
+import {
+  getTimelineResourceBudgetStatus,
+  readTimelinePerformanceDiagnostics,
+  resolveTimelineScrollStrategy,
+} from "./timelinePerformanceDiagnostics";
+import { resolveTimelineViewportBudgets } from "./timelineViewportBudgets";
+
+describe("timeline performance diagnostics", () => {
+  afterEach(() => {
+    document.body.replaceChildren();
+  });
+
+  it("reads mounted resources without mutating the timeline", () => {
+    document.body.innerHTML = `
+      <div aria-label="Timeline" data-timeline-scheduler-queued="3"
+           data-timeline-scheduler-active="2" data-timeline-cache-bytes="4096">
+        <div data-timeline-row><div data-clip="true"></div><div data-clip="true"></div></div>
+        <div data-timeline-row><div data-clip="true"></div></div>
+        <div data-timeline-grid-cell></div><div data-timeline-grid-cell></div>
+        <div data-timeline-poster-state="ready"></div>
+        <div data-timeline-poster-state="error"></div>
+      </div>`;
+    const before = document.body.innerHTML;
+
+    expect(readTimelinePerformanceDiagnostics()).toMatchObject({
+      timelineRoots: 1,
+      mountedRows: 2,
+      mountedClipRoots: 3,
+      maxMountedClipRootsInOneRow: 2,
+      mountedTimeGridCells: 2,
+      schedulerQueued: 3,
+      schedulerActive: 2,
+      cacheBytes: 4096,
+      posterStates: { idle: 0, loading: 0, ready: 1, fallback: 0, error: 1 },
+    });
+    expect(document.body.innerHTML).toBe(before);
+  });
+
+  it("returns the zero baseline after unmount or reset removes the DOM", () => {
+    document.body.innerHTML = '<div aria-label="Timeline"><div data-clip="true"></div></div>';
+    expect(readTimelinePerformanceDiagnostics().mountedClipRoots).toBe(1);
+
+    document.body.replaceChildren();
+
+    expect(readTimelinePerformanceDiagnostics()).toEqual({
+      timelineRoots: 0,
+      mountedRows: 0,
+      mountedClipRoots: 0,
+      maxMountedClipRootsInOneRow: 0,
+      mountedTimeGridCells: 0,
+      mountedTimelineDescendants: 0,
+      schedulerQueued: 0,
+      schedulerActive: 0,
+      cacheBytes: 0,
+      posterStates: { idle: 0, loading: 0, ready: 0, fallback: 0, error: 0 },
+    });
+  });
+
+  it("checks the DOM ceilings including the strict descendant boundary", () => {
+    const budgets = resolveTimelineViewportBudgets({
+      maxMountedClipRoots: 2,
+      maxMountedClipRootsPerRow: 1,
+      maxMountedTimelineDescendants: 4,
+    });
+    expect(
+      getTimelineResourceBudgetStatus(
+        {
+          ...readTimelinePerformanceDiagnostics(),
+          mountedClipRoots: 2,
+          maxMountedClipRootsInOneRow: 2,
+          mountedTimelineDescendants: 4,
+        },
+        budgets,
+      ),
+    ).toEqual({ clipRoots: true, clipRootsPerRow: false, descendants: false });
+  });
+
+  it("selects direct scrolling only through the configured safety envelope", () => {
+    expect(resolveTimelineScrollStrategy(8_000_000)).toBe("direct");
+    expect(resolveTimelineScrollStrategy(8_000_001)).toBe("segmented");
+    expect(() => resolveTimelineScrollStrategy(Number.NaN)).toThrow("content width");
+  });
+});
diff --git a/packages/studio/src/player/lib/timelinePerformanceDiagnostics.ts b/packages/studio/src/player/lib/timelinePerformanceDiagnostics.ts
new file mode 100644
index 0000000000..e1f2f5964b
--- /dev/null
+++ b/packages/studio/src/player/lib/timelinePerformanceDiagnostics.ts
@@ -0,0 +1,115 @@
+import { TIMELINE_VIEWPORT_BUDGETS, type TimelineViewportBudgets } from "./timelineViewportBudgets";
+
+export type TimelinePosterState = "idle" | "loading" | "ready" | "fallback" | "error";
+
+export interface TimelinePerformanceDiagnostics {
+  timelineRoots: number;
+  mountedRows: number;
+  mountedClipRoots: number;
+  maxMountedClipRootsInOneRow: number;
+  mountedTimeGridCells: number;
+  mountedTimelineDescendants: number;
+  schedulerQueued: number;
+  schedulerActive: number;
+  cacheBytes: number;
+  posterStates: Readonly<Record<TimelinePosterState, number>>;
+}
+
+export interface TimelineResourceBudgetStatus {
+  clipRoots: boolean;
+  clipRootsPerRow: boolean;
+  descendants: boolean;
+}
+
+function readNonNegativeNumber(value: string | undefined): number {
+  const number = Number(value);
+  return Number.isFinite(number) && number >= 0 ? number : 0;
+}
+
+function countPosters(root: ParentNode): Readonly<Record<TimelinePosterState, number>> {
+  const counts: Record<TimelinePosterState, number> = {
+    idle: 0,
+    loading: 0,
+    ready: 0,
+    fallback: 0,
+    error: 0,
+  };
+  for (const node of root.querySelectorAll<HTMLElement>("[data-timeline-poster-state]")) {
+    const state = node.dataset.timelinePosterState;
+    if (state && state in counts) counts[state as TimelinePosterState] += 1;
+  }
+  return Object.freeze(counts);
+}
+
+function maxClipsInOneRow(root: ParentNode): number {
+  const byRow = new Map<Element | null, number>();
+  for (const clip of root.querySelectorAll<HTMLElement>('[data-clip="true"]')) {
+    const row = clip.closest("[data-timeline-row]");
+    byRow.set(row, (byRow.get(row) ?? 0) + 1);
+  }
+  return Math.max(0, ...byRow.values());
+}
+
+function sumDataAttribute(root: ParentNode, selector: string, dataKey: string): number {
+  let total = 0;
+  for (const node of root.querySelectorAll<HTMLElement>(selector)) {
+    total += readNonNegativeNumber(node.dataset[dataKey]);
+  }
+  return total;
+}
+
+/**
+ * Read current timeline costs directly from the mounted DOM. No counters are
+ * retained, so an unmount or project reset is reflected as a zero baseline on
+ * the next read rather than depending on cleanup ordering.
+ */
+export function readTimelinePerformanceDiagnostics(
+  root: ParentNode = document,
+): Readonly<TimelinePerformanceDiagnostics> {
+  const timelineRoots = root.querySelectorAll<HTMLElement>('[aria-label="Timeline"]');
+  let mountedTimelineDescendants = 0;
+  for (const timelineRoot of timelineRoots) {
+    mountedTimelineDescendants += timelineRoot.querySelectorAll("*").length;
+  }
+  return Object.freeze({
+    timelineRoots: timelineRoots.length,
+    mountedRows: root.querySelectorAll("[data-timeline-row]").length,
+    mountedClipRoots: root.querySelectorAll('[data-clip="true"]').length,
+    maxMountedClipRootsInOneRow: maxClipsInOneRow(root),
+    mountedTimeGridCells: root.querySelectorAll("[data-timeline-grid-cell]").length,
+    mountedTimelineDescendants,
+    schedulerQueued: sumDataAttribute(
+      root,
+      "[data-timeline-scheduler-queued]",
+      "timelineSchedulerQueued",
+    ),
+    schedulerActive: sumDataAttribute(
+      root,
+      "[data-timeline-scheduler-active]",
+      "timelineSchedulerActive",
+    ),
+    cacheBytes: sumDataAttribute(root, "[data-timeline-cache-bytes]", "timelineCacheBytes"),
+    posterStates: countPosters(root),
+  });
+}
+
+export function getTimelineResourceBudgetStatus(
+  diagnostics: TimelinePerformanceDiagnostics,
+  budgets: Readonly<TimelineViewportBudgets> = TIMELINE_VIEWPORT_BUDGETS,
+): Readonly<TimelineResourceBudgetStatus> {
+  return Object.freeze({
+    clipRoots: diagnostics.mountedClipRoots <= budgets.maxMountedClipRoots,
+    clipRootsPerRow: diagnostics.maxMountedClipRootsInOneRow <= budgets.maxMountedClipRootsPerRow,
+    descendants: diagnostics.mountedTimelineDescendants < budgets.maxMountedTimelineDescendants,
+  });
+}
+
+export function resolveTimelineScrollStrategy(
+  contentWidthPx: number,
+  budgets: Readonly<TimelineViewportBudgets> = TIMELINE_VIEWPORT_BUDGETS,
+): "direct" | "segmented" {
+  if (!Number.isFinite(contentWidthPx) || contentWidthPx < 0) {
+    throw new RangeError("Timeline content width must be a finite non-negative number");
+  }
+  return contentWidthPx <= budgets.directScrollSafetyPx ? "direct" : "segmented";
+}
diff --git a/packages/studio/src/player/lib/timelineViewportBudgets.test.ts b/packages/studio/src/player/lib/timelineViewportBudgets.test.ts
new file mode 100644
index 0000000000..a2f1bcf018
--- /dev/null
+++ b/packages/studio/src/player/lib/timelineViewportBudgets.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from "vitest";
+import {
+  TIMELINE_VIEWPORT_BUDGETS,
+  resolveTimelineViewportBudgets,
+} from "./timelineViewportBudgets";
+
+describe("timeline viewport budgets", () => {
+  it("owns the agreed direct-scroll, DOM, media, and measurement ceilings", () => {
+    expect(TIMELINE_VIEWPORT_BUDGETS).toMatchObject({
+      directScrollSafetyPx: 8_000_000,
+      maxMountedClipRoots: 512,
+      maxMountedClipRootsPerRow: 128,
+      maxMountedTimelineDescendants: 5_000,
+      thumbnailCacheBytes: 64 * 1024 * 1024,
+      waveformCacheBytes: 16 * 1024 * 1024,
+      interactionP95Ms: 50,
+      constrainedInteractionP95Ms: 75,
+      posterCoverageRatio: 0.9,
+      supportedFixtureFallbackRatio: 0.02,
+      warmupRuns: 3,
+      measuredRuns: 5,
+      requiredPassingRuns: 4,
+    });
+    expect(Object.isFrozen(TIMELINE_VIEWPORT_BUDGETS)).toBe(true);
+  });
+
+  it("creates an immutable test override without changing production defaults", () => {
+    const resolved = resolveTimelineViewportBudgets({
+      directScrollSafetyPx: 256,
+      measuredRuns: 1,
+      requiredPassingRuns: 1,
+    });
+
+    expect(resolved.directScrollSafetyPx).toBe(256);
+    expect(resolved.maxMountedClipRoots).toBe(512);
+    expect(TIMELINE_VIEWPORT_BUDGETS.directScrollSafetyPx).toBe(8_000_000);
+    expect(Object.isFrozen(resolved)).toBe(true);
+  });
+
+  it.each([
+    [{ maxMountedClipRoots: -1 }, "maxMountedClipRoots"],
+    [{ frameIntervalP95Ms: Number.NaN }, "frameIntervalP95Ms"],
+    [{ measuredRuns: 4, requiredPassingRuns: 5 }, "requiredPassingRuns"],
+    [{ posterCoverageRatio: 1.1 }, "posterCoverageRatio"],
+  ] as const)("rejects an invalid override %#", (overrides, message) => {
+    expect(() => resolveTimelineViewportBudgets(overrides)).toThrow(message);
+  });
+});
diff --git a/packages/studio/src/player/lib/timelineViewportBudgets.ts b/packages/studio/src/player/lib/timelineViewportBudgets.ts
new file mode 100644
index 0000000000..178bb3f4b7
--- /dev/null
+++ b/packages/studio/src/player/lib/timelineViewportBudgets.ts
@@ -0,0 +1,125 @@
+export interface TimelineViewportBudgets {
+  directScrollSafetyPx: number;
+  rowOverscanPerSide: number;
+  timeOverscanViewportRatio: number;
+  maxMountedClipRoots: number;
+  maxMountedClipRootsPerRow: number;
+  maxMountedTimelineDescendants: number;
+  posterMaxPhysicalWidth: number;
+  posterMaxPhysicalHeight: number;
+  posterDprCap: number;
+  richPreviewFrameCount: number;
+  concurrentVideoDecodes: number;
+  concurrentMetadataJobs: number;
+  concurrentCompositionFetches: number;
+  concurrentServerPages: number;
+  thumbnailCacheBytes: number;
+  thumbnailCacheEntries: number;
+  thumbnailCacheEntriesPerProject: number;
+  metadataRegistryEntries: number;
+  metadataFailureTtlMs: number;
+  waveformCacheBytes: number;
+  waveformCacheEntries: number;
+  compositionDiskCacheBytes: number;
+  compositionDiskCacheMaxAgeMs: number;
+  interactionP95Ms: number;
+  frameIntervalP95Ms: number;
+  constrainedInteractionP95Ms: number;
+  constrainedFrameIntervalP95Ms: number;
+  longTaskLimitMs: number;
+  memoryReturnToleranceRatio: number;
+  posterColdP95Ms: number;
+  posterCachedP95Ms: number;
+  constrainedPosterColdP95Ms: number;
+  constrainedPosterCachedP95Ms: number;
+  posterCoverageRatio: number;
+  posterCoverageSettleMs: number;
+  constrainedPosterCoverageSettleMs: number;
+  richPreviewP95Ms: number;
+  constrainedRichPreviewP95Ms: number;
+  supportedFixtureFallbackRatio: number;
+  warmupRuns: number;
+  measuredRuns: number;
+  requiredPassingRuns: number;
+}
+
+const MEBIBYTE = 1024 * 1024;
+const DAY_MS = 24 * 60 * 60 * 1000;
+
+/**
+ * The sole default budget owner for timeline viewport and media virtualization.
+ * Consumers may resolve an immutable per-test override; production defaults are
+ * never mutated globally.
+ */
+export const TIMELINE_VIEWPORT_BUDGETS: Readonly<TimelineViewportBudgets> = Object.freeze({
+  directScrollSafetyPx: 8_000_000,
+  rowOverscanPerSide: 4,
+  timeOverscanViewportRatio: 0.5,
+  maxMountedClipRoots: 512,
+  maxMountedClipRootsPerRow: 128,
+  maxMountedTimelineDescendants: 5_000,
+  posterMaxPhysicalWidth: 240,
+  posterMaxPhysicalHeight: 135,
+  posterDprCap: 1.5,
+  richPreviewFrameCount: 6,
+  concurrentVideoDecodes: 2,
+  concurrentMetadataJobs: 4,
+  concurrentCompositionFetches: 2,
+  concurrentServerPages: 1,
+  thumbnailCacheBytes: 64 * MEBIBYTE,
+  thumbnailCacheEntries: 256,
+  thumbnailCacheEntriesPerProject: 96,
+  metadataRegistryEntries: 512,
+  metadataFailureTtlMs: 30_000,
+  waveformCacheBytes: 16 * MEBIBYTE,
+  waveformCacheEntries: 256,
+  compositionDiskCacheBytes: 512 * MEBIBYTE,
+  compositionDiskCacheMaxAgeMs: 14 * DAY_MS,
+  interactionP95Ms: 50,
+  frameIntervalP95Ms: 33.3,
+  constrainedInteractionP95Ms: 75,
+  constrainedFrameIntervalP95Ms: 50,
+  longTaskLimitMs: 50,
+  memoryReturnToleranceRatio: 0.15,
+  posterColdP95Ms: 750,
+  posterCachedP95Ms: 250,
+  constrainedPosterColdP95Ms: 1_200,
+  constrainedPosterCachedP95Ms: 400,
+  posterCoverageRatio: 0.9,
+  posterCoverageSettleMs: 1_500,
+  constrainedPosterCoverageSettleMs: 2_500,
+  richPreviewP95Ms: 750,
+  constrainedRichPreviewP95Ms: 1_200,
+  supportedFixtureFallbackRatio: 0.02,
+  warmupRuns: 3,
+  measuredRuns: 5,
+  requiredPassingRuns: 4,
+});
+
+function assertValidBudget(name: keyof TimelineViewportBudgets, value: number): void {
+  if (!Number.isFinite(value) || value < 0) {
+    throw new RangeError(`Timeline viewport budget ${name} must be a finite non-negative number`);
+  }
+}
+
+export function resolveTimelineViewportBudgets(
+  overrides: Partial<TimelineViewportBudgets> = {},
+): Readonly<TimelineViewportBudgets> {
+  for (const [name, value] of Object.entries(overrides)) {
+    assertValidBudget(name as keyof TimelineViewportBudgets, value);
+  }
+  const resolved = { ...TIMELINE_VIEWPORT_BUDGETS, ...overrides };
+  if (resolved.requiredPassingRuns > resolved.measuredRuns) {
+    throw new RangeError("Timeline viewport budget requiredPassingRuns cannot exceed measuredRuns");
+  }
+  for (const name of [
+    "memoryReturnToleranceRatio",
+    "posterCoverageRatio",
+    "supportedFixtureFallbackRatio",
+  ] as const) {
+    if (resolved[name] > 1) {
+      throw new RangeError(`Timeline viewport budget ${name} cannot exceed 1`);
+    }
+  }
+  return Object.freeze(resolved);
+}