diff --git a/public/app.css b/public/app.css
index d39c614..ac6252d 100644
--- a/public/app.css
+++ b/public/app.css
@@ -86,7 +86,9 @@ main { max-width: 680px; margin: 0 auto; }
 
 /* ---------- media ---------- */
 
-.media { border-block: 1px solid var(--line); background: #000; }
+/* Card-coloured, not black: whatever space the media does not fill should
+   read as part of the page rather than as a border around it. */
+.media { border-block: 1px solid var(--line); background: var(--card); }
 .post__head + .media, .text + .media { border-top: 1px solid var(--line); }
 
 .media__bar {
@@ -135,14 +137,21 @@ main { max-width: 680px; margin: 0 auto; }
   place-items: center;
   position: relative;
   min-width: 0;
+  min-height: 0;
 }
 
+/* Sized by its own proportions rather than stretched to the column: a
+   portrait video filling the width is mostly empty space either side of it.
+   The cap keeps any single item within the window. */
 .item img, .item video {
   display: block;
-  width: 100%;
-  max-height: 78vh;
+  width: auto;
+  height: auto;
+  max-width: 100%;
+  max-height: 70vh;
+  max-height: 70dvh;
   object-fit: contain;
-  background: #000;
+  background: transparent;
 }
 
 .item__note {
@@ -162,9 +171,31 @@ main { max-width: 680px; margin: 0 auto; }
   overflow: visible;
 }
 
-.media[data-view="grid"] .item { aspect-ratio: 1 / 1 !important; cursor: zoom-in; }
+.media[data-view="grid"] .item {
+  aspect-ratio: 1 / 1;
+  cursor: zoom-in;
+  overflow: hidden;
+}
+
+/*
+ * Filled absolutely rather than with height: 100%. The cell's height comes
+ * from its aspect-ratio, which a percentage height will not resolve against,
+ * and `aspect-ratio: auto` means "use the image's own" rather than "ignore
+ * ratios" -- between them a portrait thumbnail ends up twice the height of
+ * its cell.
+ */
 .media[data-view="grid"] .item img,
-.media[data-view="grid"] .item video { height: 100%; max-height: none; object-fit: cover; }
+.media[data-view="grid"] .item video {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  max-width: none;
+  max-height: none;
+  min-height: 0;
+  object-fit: cover;
+  aspect-ratio: auto !important;
+}
 .media[data-view="grid"] .item__note { display: none; }
 
 /* ---------- footer ---------- */
@@ -272,3 +303,30 @@ main { max-width: 680px; margin: 0 auto; }
      without hiding the rest. */
   background: color-mix(in srgb, var(--accent, #888) 7%, transparent);
 }
+
+/* ---------- fitting the window ---------- */
+
+/*
+ * On a large screen a single post should be readable without scrolling to
+ * find it -- a video especially, which you can full-screen if you want it
+ * bigger.
+ *
+ * The cap is the window less the chrome around the media: header, caption and
+ * footer come to roughly 270-350px on the posts this serves. Deriving it
+ * instead of subtracting a constant is the obvious thing to try and does not
+ * work: the card is sized by its contents, so a percentage height inside it
+ * resolves to auto and clamps nothing.
+ *
+ * A thread is a column of several posts and cannot fit whatever we do, so it
+ * keeps the looser cap above and scrolls.
+ */
+@media (min-width: 700px) and (min-height: 560px) {
+  .post:not([data-segments]) .item img,
+  .post:not([data-segments]) .item video {
+    max-height: calc(100vh - 350px);
+    max-height: calc(100dvh - 350px);
+    /* Below that the media would be too small to be worth showing; better to
+       let a very long caption scroll. */
+    min-height: 220px;
+  }
+}
diff --git a/src/render/post.ts b/src/render/post.ts
index 438ce3f..4f9dabd 100644
--- a/src/render/post.ts
+++ b/src/render/post.ts
@@ -4,6 +4,11 @@ import { html, raw, type Raw } from './html.ts';
 import { badge, layout, originalUrlBlock } from './layout.ts';
 import { linkify } from './text.ts';
 
+/**
+ * Put the ratio on the media itself, never on the figure around it. On the
+ * figure, at the full width of the column, a portrait video forces a box
+ * taller than the window and the rest of the post gets pushed off screen.
+ */
 function aspect(item: Media): string {
   return item.width && item.height ? `aspect-ratio: ${item.width} / ${item.height};` : '';
 }
@@ -13,11 +18,12 @@ function renderItem(item: Media, index: number): Raw {
 
   if (item.kind === 'video') {
     const poster = item.poster ? proxyUrlFor(item.poster) : undefined;
-    return html`<figure class="item item--video" data-index="${index}" style="${aspect(item)}">
+    return html`<figure class="item item--video" data-index="${index}">
       <video
         controls
         playsinline
         preload="metadata"
+        style="${aspect(item)}"
         ${poster ? html`poster="${poster}"` : ''}
       ><source src="${src}"></video>
       ${item.hls
@@ -26,10 +32,11 @@ function renderItem(item: Media, index: number): Raw {
     </figure>`;
   }
 
-  return html`<figure class="item item--image" data-index="${index}" style="${aspect(item)}">
+  return html`<figure class="item item--image" data-index="${index}">
     <img
       src="${src}"
       alt="${item.alt ?? ''}"
+      style="${aspect(item)}"
       loading="${index === 0 ? 'eager' : 'lazy'}"
       decoding="async"
     >