src/components/MobileFeed.tsx

@@ -76,18 +76,53 @@ function setRef<T>(ref: Ref<T> | undefined, value: T | null) {
   else if (ref) (ref as { current: T | null }).current = value;
 }
 
+/** Aspect ratio (width / height) at or below which a video fills the slide (4:5 or taller). */
+export const COVER_MAX_RATIO = 0.8;
+
+/** "cover" for portrait and near-portrait video, "contain" for wider, "cover" while the size is unknown. */
+export function videoFit(width: number, height: number): "cover" | "contain" {
+  if (!(width > 0) || !(height > 0)) return "cover";
+  return width / height <= COVER_MAX_RATIO ? "cover" : "contain";
+}
+
 function SlideMedia({ media, author }: { media: CardMedia; author: string }) {
   const box = useRef<HTMLDivElement>(null);
   const [started, setStarted] = useState(false);
+  // Real video size once metadata loads; until then the thumbnail's size decides.
+  const [videoSize, setVideoSize] = useState<[number, number] | null>(null);
+  const [thumbSize, setThumbSize] = useState<[number, number] | null>(null);
   const playable = isPlayableVideo(media);
+  useEffect(() => {
+    const video = box.current?.querySelector("video");
+    if (!video) return;
+    const read = () => {
+      if (video.videoWidth > 0 && video.videoHeight > 0) setVideoSize([video.videoWidth, video.videoHeight]);
+    };
+    read();
+    video.addEventListener("loadedmetadata", read);
+    return () => video.removeEventListener("loadedmetadata", read);
+  }, [playable, media.mp4Url, media.hlsUrl]);
   if (playable) {
+    const size = videoSize ?? thumbSize;
+    const fit = size ? videoFit(size[0], size[1]) : "cover";
     const duration = formatDuration(media.durationSeconds);
     const start = () => {
       setStarted(true);
       void box.current?.querySelector("video")?.play().catch(() => {});
     };
     return (
-      <div className="slide-media slide-video" ref={box}>
+      <div className="slide-media slide-video" data-fit={fit} ref={box}>
+        {media.src ? (
+          // The thumbnail doubles as the blurred backdrop behind a contained video, and as the size fallback.
+          // eslint-disable-next-line @next/next/no-img-element
+          <img
+            className="slide-backdrop"
+            src={media.src}
+            alt=""
+            aria-hidden="true"
+            onLoad={(e) => setThumbSize([e.currentTarget.naturalWidth, e.currentTarget.naturalHeight])}
+          />
+        ) : null}
         <div className="slide-art">
           <VideoPlayer mp4Url={media.mp4Url} hlsUrl={media.hlsUrl} poster={media.src} label={`Video from ${author}`} controls={started} />
         </div>

src/dev/FeedPreview.tsx

@@ -9,19 +9,20 @@ import { DiscoverRailsView } from "@/components/DiscoverRails";
 import { ExpertSearchView } from "@/components/ExpertSearch";
 import { FeedView } from "@/components/FeedView";
 import { toComments } from "@/lib/comment-model";
-import { toCardPosts, type CardPost } from "@/lib/feed-model";
+import { type CardPost } from "@/lib/feed-model";
 import {
   FIXTURE_CATEGORIES,
   FIXTURE_COMMENTS,
   FIXTURE_DISCOVER,
   FIXTURE_FEED_ITEMS,
+  fixtureCards,
   FIXTURE_NOW,
   FIXTURE_PERSON,
   FIXTURE_SEARCH_RESULTS,
 } from "./fixtures";
 
 export function FeedPreview() {
-  const [posts, setPosts] = useState<CardPost[]>(() => toCardPosts(FIXTURE_FEED_ITEMS));
+  const [posts, setPosts] = useState<CardPost[]>(() => fixtureCards(FIXTURE_FEED_ITEMS));
   const [draft, setDraft] = useState("");
   const [file, setFile] = useState<File | null>(null);
   const [query, setQuery] = useState("");
@@ -55,7 +56,7 @@ export function FeedPreview() {
           const it = raw as { post: { id: string } };
           return { ...it, post: { ...it.post, id: `${it.post.id}-page${next}` } };
         });
-        setPosts((cur) => [...cur, ...toCardPosts(items)]);
+        setPosts((cur) => [...cur, ...fixtureCards(items)]);
         setPage(next);
         setPaging(false);
       }, 900);
@@ -152,7 +153,7 @@ export function FeedPreview() {
             list.current?.scrollTo({ top: 0, behavior: "smooth" });
             setRefreshing(true);
             window.setTimeout(() => {
-              setPosts(toCardPosts(FIXTURE_FEED_ITEMS));
+              setPosts(fixtureCards(FIXTURE_FEED_ITEMS));
               setRefreshing(false);
             }, 600);
           }}

src/dev/fixtures.ts

@@ -2,21 +2,22 @@
 // real: no Minnect person, post, token or URL. Images are inline SVG data URIs
 // so the preview never calls the network. Later tasks extend this file.
 import type { Person } from "@/lib/auth-client";
+import { toCardPosts, type CardPost } from "@/lib/feed-model";
 import type { DiscoverCategory, DiscoverData, DiscoverExpert, DiscoverMinnect } from "@/lib/discover";
 import type { ExpertResult } from "@/lib/experts-search";
 
 /** Fixed "now" so relative times in the preview are stable. */
 export const FIXTURE_NOW = Date.parse("2026-10-19T12:00:00Z");
 
-/** A 16:9 gradient placeholder image, drawn with hsl() so no fetch happens. */
-export function placeholderImage(hue: number, label = ""): string {
+/** A gradient placeholder image (16:9 unless a size is given), drawn with hsl() so no fetch happens. */
+export function placeholderImage(hue: number, label = "", width = 1280, height = 720): string {
   const svg =
-    `<svg xmlns="http://www.w3.org/2000/svg" width="1280" height="720" viewBox="0 0 1280 720">` +
+    `<svg xmlns="http://www.w3.org/2000/svg" width="${width}" height="${height}" viewBox="0 0 ${width} ${height}">` +
     `<defs><linearGradient id="g" x1="0" y1="0" x2="1" y2="1">` +
     `<stop offset="0" stop-color="hsl(${hue} 45% 32%)"/><stop offset="1" stop-color="hsl(${(hue + 40) % 360} 40% 14%)"/>` +
-    `</linearGradient></defs><rect width="1280" height="720" fill="url(#g)"/>` +
+    `</linearGradient></defs><rect width="${width}" height="${height}" fill="url(#g)"/>` +
     (label
-      ? `<text x="64" y="660" font-family="system-ui,sans-serif" font-size="56" font-weight="700" fill="hsl(0 0% 100% / .85)">${label}</text>`
+      ? `<text x="64" y="${height - 60}" font-family="system-ui,sans-serif" font-size="56" font-weight="700" fill="hsl(0 0% 100% / .85)">${label}</text>`
       : "") +
     `</svg>`;
   return `data:image/svg+xml;charset=utf-8,${encodeURIComponent(svg)}`;
@@ -25,6 +26,22 @@ export function placeholderImage(hue: number, label = ""): string {
 /** Not a real file: preload is off, so the preview video shows only its poster, play button and length. */
 const FIXTURE_VIDEO_URL = "https://cdn.feed.sharedservices.vt.com/dev-fixture.mp4";
 
+/** Fixture videos with real (tiny) files in public/dev: the feed only accepts CDN-host mp4 URLs, so these are swapped to local paths after parsing. */
+const FIXTURE_LANDSCAPE_URL = "https://cdn.feed.sharedservices.vt.com/dev-landscape.mp4";
+const FIXTURE_PORTRAIT_URL = "https://cdn.feed.sharedservices.vt.com/dev-portrait.mp4";
+const LOCAL_FIXTURE_VIDEOS: Record<string, string> = {
+  [FIXTURE_LANDSCAPE_URL]: "/dev/landscape.mp4",
+  [FIXTURE_PORTRAIT_URL]: "/dev/portrait.mp4",
+};
+
+/** toCardPosts for the preview, with the fixture video URLs pointed at the local files. */
+export function fixtureCards(items: unknown[]): CardPost[] {
+  return toCardPosts(items).map((p) => ({
+    ...p,
+    media: p.media.map((m) => (m.mp4Url && LOCAL_FIXTURE_VIDEOS[m.mp4Url] ? { ...m, mp4Url: LOCAL_FIXTURE_VIDEOS[m.mp4Url] } : m)),
+  }));
+}
+
 /** Comments for the preview, in the shape /api/posts/[id]/comments returns. p2 holds a removed one; p4 has none. */
 export const FIXTURE_COMMENTS: Record<string, unknown[]> = {
   p1: [{ id: "c1", content: "Congratulations, the flow reads well.", createdAt: "2026-10-19T11:50:00Z", author: { displayName: "Priya Natarajan" } }],
@@ -89,6 +106,30 @@ export const FIXTURE_FEED_ITEMS: unknown[] = [
       mediaItems: [{ mediaType: "video", thumbnailUrl: placeholderImage(210, "Weekly reviews"), mp4Url: FIXTURE_VIDEO_URL, videoDuration: 154, order: 0 }],
     },
   },
+  {
+    type: "post",
+    publishedAt: "2026-10-19T07:50:00Z",
+    likesCount: 31,
+    commentsCount: 4,
+    post: {
+      id: "p8",
+      content: "Landscape recording of the quarterly walkthrough. It should show whole, centered, over a blurred copy of its own thumbnail.",
+      author: author("a8", "Tomas Reyes"),
+      mediaItems: [{ mediaType: "video", thumbnailUrl: placeholderImage(180, "Landscape 16:9"), mp4Url: FIXTURE_LANDSCAPE_URL, videoDuration: 2, order: 0 }],
+    },
+  },
+  {
+    type: "post",
+    publishedAt: "2026-10-19T07:20:00Z",
+    likesCount: 44,
+    commentsCount: 5,
+    post: {
+      id: "p9",
+      content: "Portrait clip shot on a phone. It should fill the whole slide.",
+      author: author("a9", "Ines Moreau"),
+      mediaItems: [{ mediaType: "video", thumbnailUrl: placeholderImage(300, "Portrait 9:16", 720, 1280), mp4Url: FIXTURE_PORTRAIT_URL, videoDuration: 2, order: 0 }],
+    },
+  },
   {
     type: "post",
     publishedAt: "2026-10-18T21:20:00Z",

src/styles/mobile.css

@@ -52,6 +52,22 @@
 .slide-video .slide-art video,
 .slide-video .slide-art img { display: block; width: 100%; height: 100%; object-fit: cover; }
 .slide-video .slide-art > div { height: 100%; }
+.slide-video .slide-art video { background: transparent; }
+/* Wider than 4:5: show the whole video, centered, over a blurred, darkened, enlarged copy of its own thumbnail. */
+.slide-backdrop { display: none; }
+.slide-video[data-fit="contain"] .slide-backdrop {
+  display: block;
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  transform: scale(1.25);
+  filter: blur(28px) brightness(0.5);
+  pointer-events: none;
+}
+.slide-video[data-fit="contain"] .slide-art video,
+.slide-video[data-fit="contain"] .slide-art img { object-fit: contain; }
 .slide-pending { display: grid; place-items: center; color: var(--ink-2); font-size: 14px; }
 .slide-play { z-index: 4; }
 .slide-dur { top: 128px; right: 16px; z-index: 4; }

tests/mobile-feed.test.ts

@@ -195,3 +195,14 @@ describe("top bar and progress rail", () => {
     expect(css).toMatch(/\.ph-top \{[^}]*safe-area-inset-top/);
   });
 });
+
+describe("videoFit", () => {
+  it("fills the slide for 4:5 and taller, shows whole for wider, covers while unknown", async () => {
+    const { videoFit } = await import("@/components/MobileFeed");
+    expect(videoFit(720, 1280)).toBe("cover");
+    expect(videoFit(1080, 1350)).toBe("cover");
+    expect(videoFit(1080, 1080)).toBe("contain");
+    expect(videoFit(1920, 1080)).toBe("contain");
+    expect(videoFit(0, 0)).toBe("cover");
+  });
+});