diff --git a/docs/reference/lint-rules.mdx b/docs/reference/lint-rules.mdx
index 4a099df1a4..150a7e3c26 100644
--- a/docs/reference/lint-rules.mdx
+++ b/docs/reference/lint-rules.mdx
@@ -20,7 +20,8 @@ Severities:
 
 Source: `packages/lint/src/rules/structure.ts`.
 
-The four codes marked **Warning or error** are warnings when the CLI lints a project, and errors when Studio lints the file it is editing.
+The five codes marked **Warning or error** are warnings when the CLI lints a project, and errors when Studio
+lints the file it is editing.
 
 ### `nested_structure_needs_subcomposition`
 
@@ -52,6 +53,15 @@ The four codes marked **Warning or error** are warnings when the CLI lints a pro
 
 **Fix:** Extend the root `data-duration`, or make the clip end at or before it.
 
+### `nested_clip_never_visible`
+
+**Warning or error.** A timed clip sits inside another timed element, and its time window never overlaps that
+element's, so it never appears. A nested clip's `data-start` counts from the composition's start, not from the
+element around it. Runs in sub-composition files too.
+
+**Fix:** Give the clip a `data-start` inside its parent's window, in composition time, or move it out of the
+parent.
+
 ## Composition contract
 
 Source: `packages/lint/src/rules/composition.ts`.
diff --git a/packages/cli/src/audio/analysis/targets.test.ts b/packages/cli/src/audio/analysis/targets.test.ts
index cefc9b442f..1e02812ed2 100644
--- a/packages/cli/src/audio/analysis/targets.test.ts
+++ b/packages/cli/src/audio/analysis/targets.test.ts
@@ -20,6 +20,7 @@ function row(
     host: null,
     hostRow: null,
     children: [],
+    parentRow: null,
     trackIndex: 0,
     src: "audio/a.wav",
     sourceFile: null,
diff --git a/packages/cli/src/timeline/a2Shared.ts b/packages/cli/src/timeline/a2Shared.ts
index 44ddd5e6d2..44bab9d666 100644
--- a/packages/cli/src/timeline/a2Shared.ts
+++ b/packages/cli/src/timeline/a2Shared.ts
@@ -137,6 +137,19 @@ export function diff(before: string, after: string): string {
   return output.length <= 8_000 ? output : `${output.slice(0, 7_997)}...`;
 }
 
+function isAncestorOf(ancestor: TimelineRow, row: TimelineRow, timeline: ProjectTimeline): boolean {
+  for (let pointer = row.parentRow; pointer; ) {
+    const parent = rowAt(timeline, pointer);
+    if (parent === ancestor) return true;
+    pointer = parent.parentRow;
+  }
+  return false;
+}
+
+function sameLineage(a: TimelineRow, b: TimelineRow, timeline: ProjectTimeline): boolean {
+  return isAncestorOf(a, b, timeline) || isAncestorOf(b, a, timeline);
+}
+
 function overlap(
   row: TimelineRow,
   timeline: ProjectTimeline,
@@ -146,6 +159,7 @@ function overlap(
   return allRows(timeline).find(
     (candidate) =>
       candidate !== row &&
+      !sameLineage(candidate, row, timeline) &&
       candidate.file === row.file &&
       candidate.trackIndex === row.trackIndex &&
       spansShareTime(start, end, candidate.start, candidate.end),
@@ -429,6 +443,7 @@ export function mutationConflict(
   if (verb === "duplicate") {
     const conflict = allRows(timeline).find(
       (candidate) =>
+        !sameLineage(candidate, row, timeline) &&
         candidate.file === row.file &&
         candidate.trackIndex === row.trackIndex &&
         isInsideSpan(nextStart, candidate.start, candidate.end),
diff --git a/packages/cli/src/timeline/describeProject.inPoint.test.ts b/packages/cli/src/timeline/describeProject.inPoint.test.ts
index 1caf194d04..79b95c3cd1 100644
--- a/packages/cli/src/timeline/describeProject.inPoint.test.ts
+++ b/packages/cli/src/timeline/describeProject.inPoint.test.ts
@@ -18,12 +18,12 @@ const SCENE = `<template id="scene-template"><div data-composition-id="scene" da
 beforeAll(ensureDOMParser);
 
 /** The video rows of a project made of `index` and `compositions/scene.html`. */
-async function videoRows(index: string, prefix: string) {
+async function videoRows(index: string, prefix: string, scene = SCENE) {
   const dir = mkdtempSync(join(tmpdir(), prefix));
   onTestFinished(() => rmSync(dir, { recursive: true, force: true }));
   mkdirSync(join(dir, "compositions"));
   writeFileSync(join(dir, "index.html"), index);
-  writeFileSync(join(dir, "compositions", "scene.html"), SCENE);
+  writeFileSync(join(dir, "compositions", "scene.html"), scene);
   const timeline = await describeProject(join(dir, "index.html"));
   return timeline.tracks.find((t) => t.kind === "video")!.rows;
 }
@@ -43,6 +43,17 @@ describe("a sub-composition's in-point", () => {
     ]);
   });
 
+  it("cuts a clip nested in a timed wrapper like the wrapper's own clips", async () => {
+    const videos = await videoRows(
+      `<div data-composition-id="main" data-duration="9"><div data-playback-start="5" id="half" data-composition-src="compositions/scene.html" data-start="5" data-duration="4"></div></div>`,
+      "hf-timeline-inpoint-nested-",
+      `<div data-composition-id="scene" data-duration="12"><div id="w" data-start="0" data-duration="12">
+<video id="n" src="../sample.mp4" muted data-start="4" data-duration="4" data-media-start="5"></video></div></div>`,
+    );
+
+    expect(videos.map((r) => [r.id, r.absStart, r.absEnd, r.mediaStart])).toEqual([["n", 5, 8, 6]]);
+  });
+
   it("moves each cut clip's media in-point by the part the cut skips", async () => {
     const videos = await videoRows(SPLIT_INDEX, "hf-timeline-inpoint-media-");
 
diff --git a/packages/cli/src/timeline/describeProject.test.ts b/packages/cli/src/timeline/describeProject.test.ts
index 586fb1b6c9..2585a110ca 100644
--- a/packages/cli/src/timeline/describeProject.test.ts
+++ b/packages/cli/src/timeline/describeProject.test.ts
@@ -116,6 +116,44 @@ describe("describeProject", () => {
     ]);
   });
 
+  it("lists clips nested in a timed wrapper at their playback time and says when one never shows", async () => {
+    const { rows, text } = await rowsOf(`<html><body>
+<div data-composition-id="main" data-width="1920" data-height="1080" data-duration="10">
+  <div id="shot2" data-start="5" data-duration="5" data-track-index="0">
+    <h1 id="seen" data-start="6" data-duration="2">In</h1>
+    <h1 id="lost" data-start="0" data-duration="2">Out</h1>
+  </div>
+</div></body></html>`);
+    const byId = (id: string) => rows.find((r) => r.id === id)!;
+    const at = (p: { kind: string; index: number }) =>
+      rows.find((r) => r.trackKind === p.kind && r.index === p.index)!.id;
+
+    expect(byId("shot2").children.map(at).sort()).toEqual(["lost", "seen"]);
+    expect(byId("seen")).toMatchObject({ absStart: 6, absEnd: 8, warnings: [], nested: false });
+    expect(at(byId("seen").parentRow!)).toBe("shot2");
+    expect(byId("lost").warnings).toEqual([
+      "never visible: plays 0-2s, but #shot2 only shows 5-10s; a nested data-start counts from the composition, not from #shot2",
+    ]);
+    expect(text).toContain("#lost lost 0-2s warning: never visible");
+    expect(text).not.toContain("no stable name");
+  });
+
+  it("lists a sub-composition mounted inside a timed wrapper with every clip it holds", async () => {
+    const { rows } = await rowsOf(`<html><body>
+<div data-composition-id="main" data-width="1920" data-height="1080" data-duration="10">
+  <div id="shot" data-start="0" data-duration="5" data-track-index="0">
+    <div id="scene" data-composition-src="compositions/title.html" data-start="0" data-duration="5"></div>
+  </div>
+</div></body></html>`);
+    const childIds = (id: string) =>
+      rows
+        .find((r) => r.id === id)!
+        .children.map((p) => rows.find((r) => r.trackKind === p.kind && r.index === p.index)?.id);
+
+    expect(childIds("shot")).toEqual(["scene"]);
+    expect(childIds("scene")).toEqual(["t1", "t2"]);
+  });
+
   it("reports each half's out-of-sync offset from its source pair, at the project fps", async () => {
     const { rows, text } = await rowsOf(`<html><body>
 <div data-composition-id="main" data-duration="10" data-fps="24">
diff --git a/packages/cli/src/timeline/describeProject.ts b/packages/cli/src/timeline/describeProject.ts
index efffc30bb0..cfb356b76e 100644
--- a/packages/cli/src/timeline/describeProject.ts
+++ b/packages/cli/src/timeline/describeProject.ts
@@ -19,6 +19,7 @@ import {
   type MediaTag,
 } from "@hyperframes/parsers/media-duration";
 import {
+  isSubCompositionHost,
   topLevelElements,
   trackKindOf,
   type StructureNode,
@@ -67,8 +68,10 @@ export interface TimelineRow extends ClipFact {
   host: string | null;
   /** Where that host row is (its kind's `rows[index]`), for hosts whose id is missing or repeated. */
   hostRow: RowPointer | null;
-  /** Where this row's sub-composition clips are (one level only); each is a full row in its kind's `rows`. */
+  /** Rows directly inside this one: its sub-composition's clips, or timed elements nested in it. */
   children: RowPointer[];
+  /** The timed row in the same file that contains this clip; `null` at a file's top level. */
+  parentRow: RowPointer | null;
   /** Signed frames from its `data-sync-origin` partner (+ = late); `null` unpaired or rates differ. */
   syncOffsetFrames: number | null;
   mediaStart: number | null;
@@ -80,10 +83,14 @@ export interface RowPointer {
 }
 
 /** A row before flattening: children are still row objects and refs are not assigned. */
-type ClipDraft = Omit<TimelineRow, "index" | "nested" | "host" | "hostRow" | "children"> & {
+type ClipDraft = Omit<
+  TimelineRow,
+  "index" | "nested" | "host" | "hostRow" | "children" | "parentRow"
+> & {
   elementId: string | null;
   hfId: string | null;
   children: ClipDraft[];
+  inside: ClipDraft[];
 };
 
 export interface TimelineTrack {
@@ -276,6 +283,7 @@ interface RowTiming {
   authored: number | null;
   absStart: number;
   children: ClipDraft[];
+  inside: ClipDraft[];
   duration: DurationResolution;
 }
 
@@ -291,11 +299,14 @@ async function resolveRowTiming(scope: DocScope, node: DomNode, depth: number):
     host && depth === 0
       ? await readSubComposition(host, scope, compositionOriginSeconds(absStart, el), slots)
       : [];
+  const inside = isSubCompositionHost(node)
+    ? []
+    : await Promise.all(topLevelElements(node).map((inner) => describeRow(scope, inner, depth)));
   const kind = el.tagName.toLowerCase();
   const duration = MEDIA_TAG.test(kind)
     ? await resolveMediaRowDuration(scope, el, kind as MediaTag, authored)
     : resolveContainerDuration(authored, children);
-  return { start, authored, absStart, children, duration };
+  return { start, authored, absStart, children, inside, duration };
 }
 
 function describeRowFields(scope: DocScope, node: DomNode, timing: RowTiming): ClipDraft {
@@ -326,6 +337,7 @@ function describeRowFields(scope: DocScope, node: DomNode, timing: RowTiming): C
     durationSource: timing.duration.durationSource,
     pendingReason: timing.duration.pendingReason,
     children: timing.children,
+    inside: timing.inside,
     ref: "",
     warnings: [],
     elementId: el.id || null,
@@ -384,7 +396,8 @@ function cutRow(row: ClipDraft, slots: readonly HostSlot[]): ClipDraft[] {
   if (kept.end === kept.start && row.absEnd > row.absStart) return [];
   const skipped = (kept.start - row.absStart) * (row.playbackRate ?? 1);
   const mediaStart = row.mediaStart === null ? null : roundMs(row.mediaStart + skipped);
-  return [{ ...row, absStart: roundMs(kept.start), absEnd: roundMs(kept.end), mediaStart }];
+  const inside = row.inside.flatMap((inner) => cutRow(inner, slots));
+  return [{ ...row, absStart: roundMs(kept.start), absEnd: roundMs(kept.end), mediaStart, inside }];
 }
 
 /** The file's real path when it is a regular file inside the project (symlinks resolved), else null. */
@@ -401,10 +414,14 @@ function realFileInside(projectDir: string, path: string): string | null {
 
 /** One flat, `absStart`-ordered list per kind holding every clip once; `children` become refs into those lists. */
 function flatten(top: ClipDraft[]): TimelineTrack[] {
-  const entries = [
-    ...top.map((clip) => ({ clip, host: null })),
-    ...top.flatMap((host) => host.children.map((clip) => ({ clip, host }))),
-  ];
+  type Entry = { clip: ClipDraft; host: ClipDraft | null; parent: ClipDraft | null };
+  const entries: Entry[] = [];
+  const add = (clip: ClipDraft, host: ClipDraft | null, parent: ClipDraft | null) => {
+    entries.push({ clip, host, parent });
+    for (const inner of clip.inside) add(inner, host, clip);
+    for (const child of clip.children) add(child, clip, null);
+  };
+  for (const clip of top) add(clip, null, null);
   const byKind = TRACK_ORDER.map((kind) => ({
     kind,
     entries: entries
@@ -426,19 +443,21 @@ function flatten(top: ClipDraft[]): TimelineTrack[] {
   }
   return byKind.map(({ kind, entries: kindEntries }) => ({
     kind,
-    rows: kindEntries.map(({ clip, host }, index) => {
+    rows: kindEntries.map(({ clip, host, parent }, index) => {
       const uniqueId = clip.elementId && fileIds.get(clip.file)?.get(clip.elementId) === 1;
       const ref = uniqueId
         ? `#${clip.elementId}`
         : clip.hfId
           ? `hf:${clip.hfId}`
           : `${kind}/${index}`;
-      const warnings =
-        uniqueId || clip.hfId
+      const warnings = [
+        ...(uniqueId || clip.hfId
           ? []
           : [
               'row has no stable name; add id="<name>" to the element in this file, then retry with #<name>',
-            ];
+            ]),
+        ...neverVisibleWarning(clip, parent),
+      ];
       return {
         ...clip,
         index,
@@ -447,12 +466,26 @@ function flatten(top: ClipDraft[]): TimelineTrack[] {
         nested: host !== null,
         host: host && host.id,
         hostRow: host && at(host),
-        children: clip.children.map(at),
+        children: [...clip.children, ...clip.inside].map(at),
+        parentRow: parent && at(parent),
       };
     }),
   }));
 }
 
+/** The runtime shows a nested clip only inside its parent's window, and its data-start counts from the composition. */
+export const NEVER_VISIBLE = "never visible";
+
+function neverVisibleWarning(clip: ClipDraft, parent: ClipDraft | null): string[] {
+  if (!parent || clip.kind === "audio") return [];
+  if (clip.absEnd <= clip.absStart || parent.absEnd <= parent.absStart) return [];
+  if (clip.absEnd > parent.absStart && clip.absStart < parent.absEnd) return [];
+  const name = parent.elementId ? `#${parent.elementId}` : `its parent <${parent.kind}>`;
+  return [
+    `${NEVER_VISIBLE}: plays ${clip.absStart}-${clip.absEnd}s, but ${name} only shows ${parent.absStart}-${parent.absEnd}s; a nested data-start counts from the composition, not from ${name}`,
+  ];
+}
+
 function projectFps(source: string): number {
   const parsed = parseFpsWithDefault(readCompositionFps(source) ?? undefined);
   return fpsToNumber(parsed.ok ? parsed.value : { num: 30, den: 1 });
diff --git a/packages/cli/src/timeline/formatTimeline.ts b/packages/cli/src/timeline/formatTimeline.ts
index 99481dac0f..564436dfc2 100644
--- a/packages/cli/src/timeline/formatTimeline.ts
+++ b/packages/cli/src/timeline/formatTimeline.ts
@@ -1,4 +1,9 @@
-import type { ProjectTimeline, TimelineRow, TimelineTrack } from "./describeProject.js";
+import {
+  NEVER_VISIBLE,
+  type ProjectTimeline,
+  type TimelineRow,
+  type TimelineTrack,
+} from "./describeProject.js";
 
 const BAR_WIDTH = 40;
 const n = (v: number) => String(Math.round(v * 1000) / 1000);
@@ -48,6 +53,7 @@ function details(row: TimelineRow): string {
     row.sourceFile && !row.children.length && "children=unread",
     row.laneError && `lanes unreadable: ${row.laneError}`,
     ...lanes,
+    ...row.warnings.filter((w) => w.startsWith(NEVER_VISIBLE)).map((w) => `warning: ${w}`),
   ]
     .filter(Boolean)
     .join(" ");
@@ -63,7 +69,7 @@ function line(row: TimelineRow, total: number): string {
 }
 
 function header({ kind, rows }: TimelineTrack): string {
-  const nested = rows.filter((r) => r.nested).length;
+  const nested = rows.filter((r) => r.nested || r.parentRow).length;
   const split = nested ? `: ${rows.length - nested} top-level, ${nested} nested` : "";
   return `${kind} (${rows.length}${split})`;
 }
diff --git a/packages/cli/src/timeline/timeExpr.test.ts b/packages/cli/src/timeline/timeExpr.test.ts
index dadc9eed52..363579f8f2 100644
--- a/packages/cli/src/timeline/timeExpr.test.ts
+++ b/packages/cli/src/timeline/timeExpr.test.ts
@@ -20,6 +20,7 @@ const row: TimelineRow = {
   host: null,
   hostRow: null,
   children: [],
+  parentRow: null,
   trackIndex: 0,
   src: null,
   sourceFile: null,
diff --git a/packages/cli/src/timeline/timeline.e2e.test.ts b/packages/cli/src/timeline/timeline.e2e.test.ts
index 5055690b7a..feec81d936 100644
--- a/packages/cli/src/timeline/timeline.e2e.test.ts
+++ b/packages/cli/src/timeline/timeline.e2e.test.ts
@@ -248,6 +248,27 @@ describe("timeline edit command", () => {
   };
   const endOf = (html: string, id: string) =>
     Number(clip(html, id).start) + Number(clip(html, id).duration);
+
+  it.each([
+    ["moves a hidden clip into its wrapper's window", ["move", "#lost", "8"], "lost", "8"],
+    ["moves a wrapper without colliding with its own clips", ["move", "#shot2", "4"], "shot2", "4"],
+    ["duplicates a clip inside its wrapper", ["duplicate", "#seen"], "seen", "6"],
+  ])("%s", (_name, args, id, start) => {
+    const dir = mkdtempSync(join(tmpdir(), "hf-timeline-nested-"));
+    writeFileSync(
+      join(dir, "index.html"),
+      `<div data-composition-id="main" data-duration="12"><div id="shot2" data-hf-id="shot2" data-start="5" data-duration="5" data-track-index="0"><div id="seen" data-hf-id="seen" data-start="6" data-duration="2">In</div><div id="lost" data-hf-id="lost" data-start="0" data-duration="2">Out</div></div></div>`,
+    );
+    try {
+      const result = run(dir, ...args);
+      expect(result.stderr).not.toContain("overlap");
+      expect(result.status).toBe(0);
+      expect(clip(readFileSync(join(dir, "index.html"), "utf8"), id).start).toBe(start);
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   const visibleAt = (html: string, time: number, ids: string[]) =>
     ids.filter((id) => isClipVisibleAt(time, Number(clip(html, id).start), endOf(html, id), 40));
 
diff --git a/packages/lint/src/rules/structure.catalog.test.ts b/packages/lint/src/rules/structure.catalog.test.ts
index 686308a956..dd6ac4ca4a 100644
--- a/packages/lint/src/rules/structure.catalog.test.ts
+++ b/packages/lint/src/rules/structure.catalog.test.ts
@@ -10,6 +10,7 @@ const STRUCTURE_CODES = new Set([
   "caption_track_kind_missing",
   "multiple_caption_tracks",
   "clip_ends_past_root_duration",
+  "nested_clip_never_visible",
 ]);
 
 function htmlFiles(dir: string): string[] {
diff --git a/packages/lint/src/rules/structure.test.ts b/packages/lint/src/rules/structure.test.ts
index e08fcd5789..618e102726 100644
--- a/packages/lint/src/rules/structure.test.ts
+++ b/packages/lint/src/rules/structure.test.ts
@@ -15,10 +15,34 @@ const STRUCTURE = new Set([
   "caption_track_kind_missing",
   "multiple_caption_tracks",
   "clip_ends_past_root_duration",
+  "nested_clip_never_visible",
 ]);
 const has = (found: Awaited<ReturnType<typeof codes>>, code: string) =>
   found.some((f) => f.code === code);
 
+describe("nested_clip_never_visible", () => {
+  const shot = (clipStart: number) =>
+    `<div id="shot2" class="clip" data-start="5" data-duration="5"><video id="v1" class="clip" src="a.mp4" data-start="${clipStart}" data-duration="2"></video></div>`;
+  const neverVisible = (found: Awaited<ReturnType<typeof codes>>) =>
+    found.filter((f) => f.code === "nested_clip_never_visible");
+
+  it("flags a nested clip whose window misses its parent's", async () => {
+    const [finding] = neverVisible(await codes(shot(0)));
+    expect(finding).toMatchObject({ elementId: "v1", severity: "warning" });
+    expect(finding?.message).toContain("only shows from 5s to 10s");
+  });
+  it("leaves a nested clip that plays inside its parent's window", async () => {
+    expect(neverVisible(await codes(shot(6)))).toEqual([]);
+  });
+  it("leaves nested audio alone: it plays whether or not its parent shows", async () => {
+    const body = `<div id="shot2" class="clip" data-start="5" data-duration="5"><audio id="a1" class="clip" src="a.mp3" data-start="0" data-duration="2"></audio></div>`;
+    expect(neverVisible(await codes(body))).toEqual([]);
+  });
+  it("checks sub-composition files too", async () => {
+    expect(neverVisible(await codes(shot(0), { isSubComposition: true }))).toHaveLength(1);
+  });
+});
+
 describe("nested_structure_needs_subcomposition", () => {
   it("flags a timed div that contains another div", async () => {
     const found = await codes(
diff --git a/packages/lint/src/rules/structure.ts b/packages/lint/src/rules/structure.ts
index 7479bf5505..6b37443a51 100644
--- a/packages/lint/src/rules/structure.ts
+++ b/packages/lint/src/rules/structure.ts
@@ -61,6 +61,7 @@ const NODE_ATTRS = [
   "data-track-kind",
   "data-composition-id",
   "data-composition-src",
+  "data-hf-media-start-basis",
 ];
 
 function toNode(open: OpenTag): TagNode {
@@ -193,6 +194,40 @@ function clipsPastRootFindings(root: TagNode, rows: TagNode[]): HyperframeLintFi
   });
 }
 
+function followsParentVisibility(clip: TagNode): boolean {
+  return clip.tag !== "audio" && clip.attrs["data-hf-media-start-basis"] !== "global";
+}
+
+function isTimed(timing: {
+  start: number | null;
+  end: number | null;
+}): timing is { start: number; end: number } {
+  return timing.start !== null && timing.end !== null;
+}
+
+/** The runtime shows a nested clip only while its parent shows, and both data-starts count from the composition. */
+function neverVisibleNestedFindings(parent: TagNode, severity: Severity): HyperframeLintFinding[] {
+  const window = readClipTiming({ getAttribute: (name) => parent.attrs[name] ?? null });
+  return topLevelElements(parent).flatMap((clip) => {
+    const inner = isSubCompositionHost(clip) ? [] : neverVisibleNestedFindings(clip, severity);
+    const own = readClipTiming({ getAttribute: (name) => clip.attrs[name] ?? null });
+    if (!followsParentVisibility(clip) || !isTimed(window) || !isTimed(own)) return inner;
+    if (own.end > window.start && own.start < window.end) return inner;
+    const shows = `${hundredths(window.start)}s to ${hundredths(window.end)}s`;
+    return [
+      {
+        code: "nested_clip_never_visible",
+        severity,
+        message: `${describe(clip)} plays from ${hundredths(own.start)}s to ${hundredths(own.end)}s, but it sits inside ${describe(parent)}, which only shows from ${shows}, so it never appears. A nested data-start counts from the composition's start, not from ${describe(parent)}.`,
+        elementId: clip.attrs.id,
+        fixHint: `Give ${describe(clip)} a data-start between ${shows} in composition time, or move it out of ${describe(parent)}.`,
+        snippet: truncateSnippet(clip.open.raw),
+      },
+      ...inner,
+    ];
+  });
+}
+
 export const structureRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
   (ctx) => {
     // The timeline shows the root composition's rows; a sub-composition file is the leaf where layout lives.
@@ -208,4 +243,12 @@ export const structureRules: Array<(ctx: LintContext) => HyperframeLintFinding[]
       ...clipsPastRootFindings(root, rows),
     ];
   },
+  (ctx) => {
+    const root = buildTree(ctx);
+    if (!root) return [];
+    const severity = ctx.options.host === "studio" ? "error" : "warning";
+    return topLevelElements(root)
+      .filter((row) => !isSubCompositionHost(row))
+      .flatMap((row) => neverVisibleNestedFindings(row, severity));
+  },
 ];
diff --git a/skills-manifest.json b/skills-manifest.json
index a877996429..b35d232477 100644
--- a/skills-manifest.json
+++ b/skills-manifest.json
@@ -34,7 +34,7 @@
       "files": 11
     },
     "hyperframes-core": {
-      "hash": "010a65915fc7b45e",
+      "hash": "66fc3e623a910923",
       "files": 11
     },
     "hyperframes-creative": {
diff --git a/skills/hyperframes-core/SKILL.md b/skills/hyperframes-core/SKILL.md
index 97e68da11f..3f6c6d1cd4 100644
--- a/skills/hyperframes-core/SKILL.md
+++ b/skills/hyperframes-core/SKILL.md
@@ -87,6 +87,9 @@ Surfaced here; full rationale in the linked reference. Do not violate:
 - Match existing composition IDs and timeline keys.
 - Adding a clip: set its `data-start`/`data-duration` intentionally against the clips around it. `data-track-index` is a Studio display lane, not a timing constraint, so it does not need to be free.
 - A clip that ends past the root `data-duration` is cut off: extend the root `data-duration` to the clip's end in the same edit (`lint` warns `clip_ends_past_root_duration`).
+- A clip nested inside another timed element counts its `data-start` from the composition, not from that
+  element, and shows only while that element shows. `timeline` lists it under its parent and warns
+  `never visible` when the windows miss (`lint` reports `nested_clip_never_visible`).
 - `data-hidden` on any composition element hides it in BOTH preview and render, overriding its time window; it is non-destructive/reversible and toggled by Studio's timeline eye icon.
 - Adding a sub-composition: verify its internal `data-composition-id` before wiring the host.