diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index ea58f0d..0842ede 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -15,6 +15,7 @@ import "../styles/feed.css";
 import "../styles/trending.css";
 import "../styles/search.css";
 import "../styles/mobile.css";
+import "../styles/sheet.css";
 import "../styles/legacy.css";
 
 const manrope = Manrope({
diff --git a/src/components/BottomSheet.tsx b/src/components/BottomSheet.tsx
new file mode 100644
index 0000000..4ac05a9
--- /dev/null
+++ b/src/components/BottomSheet.tsx
@@ -0,0 +1,90 @@
+"use client";
+
+// Bottom sheet (spec "Sheets"): slides up over a dimmed, blurred backdrop with a
+// grab handle, a serif title and a close button. Reused by the comments,
+// composer and Discover sheets. Styles live in src/styles/sheet.css.
+import { useEffect, useId, useRef, type ReactNode } from "react";
+
+const FOCUSABLE = 'button:not(:disabled), a[href], input:not(:disabled), textarea:not(:disabled), select:not(:disabled), [tabindex]:not([tabindex="-1"])';
+
+export interface BottomSheetProps {
+  title: string;
+  onClose: () => void;
+  children: ReactNode;
+  /** Extra class on the sheet, for per-sheet sizing. */
+  className?: string;
+}
+
+export function BottomSheet({ title, onClose, children, className }: BottomSheetProps) {
+  const sheet = useRef<HTMLElement | null>(null);
+  const titleId = useId();
+  const closeRef = useRef(onClose);
+  closeRef.current = onClose;
+
+  useEffect(() => {
+    const el = sheet.current;
+    if (!el) return;
+    const opener = document.activeElement as HTMLElement | null;
+    // Focus moves into the sheet: its first field, else the sheet itself.
+    const first = el.querySelector<HTMLElement>("input, textarea") ?? el.querySelector<HTMLElement>(FOCUSABLE);
+    (first ?? el).focus({ preventScroll: true });
+
+    const prevOverflow = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+
+    function onKey(e: KeyboardEvent) {
+      if (e.key === "Escape") {
+        // An open emoji popover closes first; it handles Escape itself.
+        if (el?.querySelector('[aria-haspopup="dialog"][aria-expanded="true"]')) return;
+        e.preventDefault();
+        closeRef.current();
+        return;
+      }
+      if (e.key !== "Tab" || !el) return;
+      const items = [...el.querySelectorAll<HTMLElement>(FOCUSABLE)].filter((n) => n.offsetParent !== null || n === document.activeElement);
+      if (!items.length) return;
+      const a = items[0];
+      const z = items[items.length - 1];
+      if (e.shiftKey && (document.activeElement === a || document.activeElement === el)) {
+        e.preventDefault();
+        z.focus();
+      } else if (!e.shiftKey && document.activeElement === z) {
+        e.preventDefault();
+        a.focus();
+      }
+    }
+    document.addEventListener("keydown", onKey);
+    return () => {
+      document.removeEventListener("keydown", onKey);
+      document.body.style.overflow = prevOverflow;
+      // Focus returns to the opener (when it is still on the page).
+      if (opener && opener.isConnected) opener.focus({ preventScroll: true });
+    };
+  }, []);
+
+  return (
+    <>
+      <button type="button" className="sheet-bg" aria-label="Close" tabIndex={-1} onClick={onClose} data-testid="sheet-backdrop" />
+      <section
+        ref={sheet}
+        className={`sheet${className ? ` ${className}` : ""}`}
+        role="dialog"
+        aria-modal="true"
+        aria-labelledby={titleId}
+        tabIndex={-1}
+        data-testid="bottom-sheet"
+      >
+        <span className="sheet-grab" aria-hidden="true" />
+        <div className="sheet-h">
+          <h2 id={titleId} className="type-title type-title-strong">{title}</h2>
+          <button type="button" className="sheet-x" aria-label="Close" onClick={onClose}>
+            <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.7" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+              <path d="M6 6l12 12M18 6 6 18" />
+            </svg>
+          </button>
+        </div>
+        <div className="sheet-body">{children}</div>
+      </section>
+    </>
+  );
+}
diff --git a/src/components/MobileFeed.tsx b/src/components/MobileFeed.tsx
index f6c581c..f68710f 100644
--- a/src/components/MobileFeed.tsx
+++ b/src/components/MobileFeed.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useRef, useState, type MouseEvent, type Ref } from "react";
+import { useCallback, useEffect, useRef, useState, type MouseEvent, type Ref } from "react";
 import { canDelete, formatDuration, isPlayableVideo, relativeTime, type CardMedia, type CardPost } from "@/lib/feed-model";
 import { GrowButton } from "./Buttons";
 import { HexAvatar } from "./HexAvatar";
@@ -8,6 +8,9 @@ import { PlayIcon, PostMenu } from "./PostCard";
 import { VideoPlayer } from "./VideoPlayer";
 import { FeaturedCarousel } from "./FeaturedCarousel";
 import { MobileDock } from "./MobileDock";
+import { BottomSheet } from "./BottomSheet";
+import { CommentsSection } from "./CommentsSection";
+import { Composer } from "./Composer";
 import type { DiscoverExpert } from "@/lib/discover";
 import type { FeedViewProps } from "./FeedView";
 
@@ -278,6 +281,17 @@ function LoadSlide({ index, current, sentinelRef }: { index: number; current: bo
 export function MobileFeed(p: FeedViewProps) {
   const reel = useRef<HTMLDivElement | null>(null);
   const [current, setCurrent] = useState(0);
+  // The open sheet: the comments of one post, the composer, or none.
+  const [sheet, setSheet] = useState<{ kind: "comments"; postId: string } | { kind: "composer" } | null>(null);
+  const closeSheet = useCallback(() => setSheet(null), []);
+  // A successful post closes the composer: publishing ends with no error and an emptied draft.
+  const wasBusy = useRef(false);
+  const composerBusy = !!p.composer?.busy;
+  const composerClear = !!p.composer && !p.composer.error && !p.composer.value && !p.composer.file;
+  useEffect(() => {
+    if (wasBusy.current && !composerBusy && composerClear) setSheet((s) => (s?.kind === "composer" ? null : s));
+    wasBusy.current = composerBusy;
+  }, [composerBusy, composerClear]);
   const onCurrentChange = p.onCurrentChange;
   const showLoad = p.status === "loading" || (p.status === "ready" && !p.loadMoreFailed && (p.loadingMore || !!p.hasMore));
   const experts = p.featuredExperts ?? [];
@@ -421,11 +435,27 @@ export function MobileFeed(p: FeedViewProps) {
           slide={{ index, kind, post: currentPost }}
           onToggleLike={p.onToggleLike}
           likeBusy={currentPost ? p.likeBusy?.has(currentPost.key) : false}
-          onOpenComments={p.onOpenComments}
-          onOpenComposer={p.onOpenComposer}
+          onOpenComments={(postId) => {
+            setSheet({ kind: "comments", postId });
+            p.onOpenComments?.(postId);
+          }}
+          onOpenComposer={() => {
+            setSheet({ kind: "composer" });
+            p.onOpenComposer?.();
+          }}
           onOpenMore={p.onOpenMore}
         />
       ) : null}
+      {sheet?.kind === "comments" ? (
+        <BottomSheet title="Comments" onClose={closeSheet} className="sheet-comments">
+          <CommentsSection postId={sheet.postId} onSignedOut={p.onSignedOut} />
+        </BottomSheet>
+      ) : null}
+      {sheet?.kind === "composer" && p.composer ? (
+        <BottomSheet title="New post" onClose={closeSheet} className="sheet-composer">
+          <Composer {...p.composer} />
+        </BottomSheet>
+      ) : null}
     </section>
   );
 }
diff --git a/src/styles/sheet.css b/src/styles/sheet.css
new file mode 100644
index 0000000..bde550b
--- /dev/null
+++ b/src/styles/sheet.css
@@ -0,0 +1,93 @@
+/*
+ * Bottom sheets (spec "Sheets"). Ported from the mockup's .psheet, .grab,
+ * .psheet-h and .ph-bg rules, plus the `.mx.sheet .cmp` composer rules. Tokens
+ * only. The sheet sits inside the phone feed (.reel-feed), over the dock.
+ */
+@keyframes sheet-up { from { transform: translateY(100%); } to { transform: none; } }
+@keyframes sheet-fade { from { opacity: 0; } to { opacity: 1; } }
+
+.sheet-bg {
+  position: absolute;
+  inset: 0;
+  z-index: 55;
+  appearance: none;
+  border: 0;
+  padding: 0;
+  cursor: pointer;
+  background: color-mix(in srgb, var(--bg), transparent 50%);
+  -webkit-backdrop-filter: blur(3px);
+  backdrop-filter: blur(3px);
+  animation: sheet-fade .3s both;
+}
+.sheet {
+  position: absolute;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  z-index: 60;
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+  max-height: 78%;
+  padding: 10px 14px calc(24px + env(safe-area-inset-bottom, 0px));
+  background: var(--bg);
+  color: var(--ink);
+  border-radius: 26px 26px 0 0;
+  box-shadow: 0 -20px 50px -10px color-mix(in srgb, var(--neu-d), transparent 45%), inset 0 1px 0 var(--line);
+  animation: sheet-up .45s cubic-bezier(.2,.9,.25,1) both;
+  outline: none;
+}
+.sheet-grab {
+  flex: none;
+  width: 40px;
+  height: 5px;
+  margin: 0 auto 4px;
+  border-radius: 3px;
+  background: var(--line-2);
+}
+.sheet-h { flex: none; display: flex; align-items: center; justify-content: space-between; gap: 8px; }
+.sheet-h h2 { margin: 0; font-size: 25px; line-height: 1.2; }
+.sheet-x {
+  appearance: none;
+  flex: none;
+  width: 44px;
+  height: 44px;
+  margin: 0;
+  padding: 0;
+  border: 0;
+  border-radius: var(--r-ctl);
+  display: grid;
+  place-items: center;
+  background: transparent;
+  color: var(--ink-2);
+  cursor: pointer;
+  transition: background-color .2s, color .2s;
+}
+.sheet-x svg { width: 20px; height: 20px; }
+@media (hover: hover) { .sheet-x:hover { background: var(--tile-2); color: var(--ink); } }
+.sheet-x:focus-visible { outline: 2px solid var(--ink); outline-offset: 2px; }
+.sheet-body { min-height: 0; display: flex; flex-direction: column; gap: 10px; overflow-y: auto; }
+
+/* Comments sheet: the shared comments panel without its inline-card chrome. */
+.sheet-comments .post-cm { margin: 0; padding: 0; border-top: 0; animation: none; flex: 1; min-height: 0; }
+.sheet-comments .post-cm-list { overflow-y: auto; min-height: 60px; }
+.sheet-comments .post-cm-form { flex: none; }
+.sheet-comments .post-cm-form input { font-size: 16px; }
+
+/* Composer sheet: the desktop composer, borderless, with a 150px text area. */
+.sheet-composer { max-height: 92%; }
+.sheet-composer .sheet-body { overflow: visible; }
+.sheet-composer .composer,
+.sheet-composer .composer.composer-open { margin: 0; padding: 0; background: transparent; box-shadow: none; }
+.sheet-composer .composer textarea,
+.sheet-composer .composer.composer-open textarea { height: 150px; font-size: 16px; }
+.sheet-composer .composer-bar { flex-wrap: wrap; }
+.sheet-composer .composer-kbd { display: none; }
+/* The popover anchors to the whole composer (not the emoji button) so it fits a phone: full width, just above the toolbar. */
+.sheet-composer .composer-emoji-wrap { position: static; }
+.sheet-composer .composer-emoji { left: 0; right: 0; width: auto; top: auto; bottom: 64px; }
+.sheet-composer .composer-att.composer-att-indent { margin-left: 0; }
+
+@media (prefers-reduced-motion: reduce) {
+  .sheet, .sheet-bg { animation: none !important; }
+}
diff --git a/tests/bottom-sheet.test.ts b/tests/bottom-sheet.test.ts
new file mode 100644
index 0000000..7261808
--- /dev/null
+++ b/tests/bottom-sheet.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { createElement, type FC } from "react";
+import { renderToStaticMarkup } from "react-dom/server";
+import { readFileSync } from "node:fs";
+import { BottomSheet } from "@/components/BottomSheet";
+
+const css = readFileSync("src/styles/sheet.css", "utf8");
+const feed = readFileSync("src/components/MobileFeed.tsx", "utf8");
+
+describe("BottomSheet", () => {
+  const html = renderToStaticMarkup(createElement(BottomSheet as FC<{ title: string; onClose: () => void }>, { title: "New post", onClose: () => {} }, createElement("p", null, "body")));
+  it("is a modal dialog named by its serif title, with a grab handle, close buttons and a backdrop", () => {
+    expect(html).toContain('role="dialog"');
+    expect(html).toContain('aria-modal="true"');
+    expect(html).toMatch(/aria-labelledby="([^"]+)"[\s\S]*<h2 id="\1"[^>]*type-title[^>]*>New post<\/h2>/);
+    expect(html).toContain("sheet-grab");
+    expect(html).toContain('class="sheet-bg"');
+    expect(html.match(/aria-label="Close"/g)?.length).toBe(2);
+    expect(html).toContain("<p>body</p>");
+  });
+  it("slides up over 450ms and turns the motion off under reduced motion", () => {
+    expect(css).toMatch(/animation: sheet-up \.45s/);
+    expect(css).toMatch(/prefers-reduced-motion: reduce[\s\S]*animation: none/);
+    expect(css).toMatch(/backdrop-filter: blur\(3px\)/);
+  });
+  it("gives the composer sheet a 150px text area", () => {
+    expect(css).toMatch(/\.sheet-composer \.composer textarea[\s\S]*height: 150px/);
+  });
+});
+
+describe("phone feed sheets", () => {
+  it("opens the shared comments section and composer from the dock", () => {
+    expect(feed).toContain('<BottomSheet title="Comments"');
+    expect(feed).toContain("<CommentsSection postId={sheet.postId}");
+    expect(feed).toContain('<BottomSheet title="New post"');
+    expect(feed).toContain("<Composer {...p.composer} />");
+  });
+});