diff --git a/packages/review/app/src/animation/animation-block.tsx b/packages/review/app/src/animation/animation-block.tsx
index e3fb0d692..94fdf2e72 100644
--- a/packages/review/app/src/animation/animation-block.tsx
+++ b/packages/review/app/src/animation/animation-block.tsx
@@ -68,6 +68,7 @@ import type { PlayerStop, StagePlayer } from "./stage-player";
 import {
   type HostMessage,
   STAGE_BOOT,
+  STAGE_MIN_WIDTH,
   type SlotLayout,
   type StageColorScheme,
   type StageInit,
@@ -301,9 +302,10 @@ function StageBlock({
   const { diagramFullscreenOnClick } = useWhiteboardSettings();
   const frameRef = useRef<HTMLIFrameElement>(null);
   const [contentHeight, setContentHeight] = useState(STAGE_MIN_HEIGHT);
-  // The widest the stage's content has reached past its edge, in any frame;
-  // the stage keeps that width and a narrower block scrolls it.
-  const [minWidth, setMinWidth] = useState(0);
+  // The board's reading width, or the widest the stage's content has
+  // reached past its edge in any frame: the stage keeps that width and a
+  // narrower block scrolls it.
+  const [minWidth, setMinWidth] = useState(STAGE_MIN_WIDTH);
   // The tallest frame of the scenario, from a hidden measuring copy, so the
   // stage does not change height as the reader steps through frames.
   const [measuredHeight, setMeasuredHeight] = useState(0);
@@ -360,7 +362,7 @@ function StageBlock({
   // of its height, and the width its stage was laid out at.
   const holdInline = () => {
     setHeld(figureRef.current?.offsetHeight ?? 0);
-    setInlineWidth(canvasRef.current?.clientWidth ?? 0);
+    setInlineWidth(Math.max(canvasRef.current?.clientWidth ?? 0, minWidth));
   };
 
   const openRef = useRef<HTMLButtonElement>(null);
@@ -1100,11 +1102,7 @@ function StageBlock({
         >
           <div
             ref={canvasRef}
-            {...stylex.props(
-              styles.canvas,
-              minWidth > 0 && styles.canvasScrolls,
-              scaled && styles.canvasScaled,
-            )}
+            {...stylex.props(styles.canvas, scaled && styles.canvasScaled)}
             style={{
               height: stageHeight,
               ...(scaled && {
@@ -1113,10 +1111,7 @@ function StageBlock({
               }),
             }}
           >
-            <div
-              {...stylex.props(styles.sheet)}
-              style={{ minWidth: minWidth || undefined }}
-            >
+            <div {...stylex.props(styles.sheet)} style={{ minWidth }}>
               <iframe
                 key={stageKey}
                 ref={frameRef}
@@ -1476,12 +1471,10 @@ const styles = stylex.create({
   canvas: {
     position: "relative",
     flex: "0 0 auto",
-    overflow: "hidden",
-  },
-  // The stage asked for a width: it keeps it, and a narrower block scrolls
-  // it sideways rather than folding the drawing.
-  canvasScrolls: {
+    // The stage keeps its width, and a narrower block scrolls it sideways
+    // rather than folding the drawing.
     overflowX: "auto",
+    overflowY: "hidden",
   },
   canvasScaled: {
     transformOrigin: "0 0",
diff --git a/packages/review/app/src/animation/stage-protocol.ts b/packages/review/app/src/animation/stage-protocol.ts
index 449fe1e2d..c70db446e 100644
--- a/packages/review/app/src/animation/stage-protocol.ts
+++ b/packages/review/app/src/animation/stage-protocol.ts
@@ -116,6 +116,11 @@ export type HostMessage =
 /** The tallest an auto-sized stage may report itself. */
 export const STAGE_MAX_HEIGHT = 4000;
 
+/** The narrowest a stage lays out: the board's reading width. In a
+ * narrower block it keeps this width and scrolls sideways, rather than
+ * squeezing a drawing made for the board. */
+export const STAGE_MIN_WIDTH = 720;
+
 /** The widest a stage may ask to stay, scrolling sideways in a narrower
  * block. */
 export const STAGE_MAX_WIDTH = 4000;
diff --git a/packages/review/app/src/animation/stage-runtime.ts b/packages/review/app/src/animation/stage-runtime.ts
index 1a7f8576f..76a224d96 100644
--- a/packages/review/app/src/animation/stage-runtime.ts
+++ b/packages/review/app/src/animation/stage-runtime.ts
@@ -1783,6 +1783,51 @@ export function stageRuntime(port: MessagePort): void {
         : [],
     );
 
+    // Every drawn line, as thin boxes along its curve: a label clear of
+    // cards can still sit on a parallel edge fanned a few pixels from its
+    // own, so it avoids every line but its own.
+    const lines = new Map<ArrowRecord, DOMRect[]>();
+
+    for (const arrow of arrows) {
+      const route = arrow.labelRoute;
+
+      if (
+        !route ||
+        (arrow.group.dataset.wbState && arrow.group.dataset.wbState !== "on") ||
+        !arrow.path.checkVisibility({
+          visibilityProperty: true,
+          opacityProperty: true,
+        })
+      )
+        continue;
+
+      const { start, control, end } = route;
+      const boxes: DOMRect[] = [];
+      let previous = start;
+
+      for (let step = 1; step <= 24; step++) {
+        const t = step / 24;
+        const u = 1 - t;
+
+        const point = {
+          x: u * u * start.x + 2 * u * t * control.x + t * t * end.x,
+          y: u * u * start.y + 2 * u * t * control.y + t * t * end.y,
+        };
+
+        boxes.push(
+          new DOMRect(
+            Math.min(previous.x, point.x) - 1,
+            Math.min(previous.y, point.y) - 1,
+            Math.abs(point.x - previous.x) + 2,
+            Math.abs(point.y - previous.y) + 2,
+          ),
+        );
+        previous = point;
+      }
+
+      lines.set(arrow, boxes);
+    }
+
     for (const arrow of arrows) {
       const { text, labelRoute } = arrow;
 
@@ -1884,6 +1929,19 @@ export function stageRuntime(port: MessagePort): void {
       }
 
       const collisions = [...obstacles, ...endpoints, ...controls, ...flights];
+
+      // Only the lines near enough to the label's candidates to meet them.
+      const reach = new DOMRect(
+        baseX + relative.left - 150,
+        baseY + relative.top - 160,
+        relative.width + 300,
+        relative.height + 320,
+      );
+
+      const crossing = [...lines]
+        .filter(([other]) => other !== arrow)
+        .flatMap(([, boxes]) => boxes.filter((box) => overlaps(box, reach)));
+
       let selected: { x: number; y: number; rect: DOMRect } | undefined;
 
       for (const [normalOffset, tangentOffset] of offsets) {
@@ -1906,6 +1964,7 @@ export function stageRuntime(port: MessagePort): void {
         if (
           inViewport &&
           !collisions.some((obstacle) => overlaps(rect, obstacle, 3)) &&
+          !crossing.some((line) => overlaps(rect, line)) &&
           !placed.some((other) => overlaps(rect, other, 4))
         ) {
           selected = { x, y, rect };
@@ -2739,20 +2798,29 @@ export function stageRuntime(port: MessagePort): void {
       grid.style.columnGap = `${Math.min(160, Math.max(floor, Math.ceil(widest) + 16))}px`;
     };
 
-    // The scene keeps its columns at any width: narrower than its cards
-    // need, it overflows the stage, which asks its host for the width and is
-    // scrolled sideways, rather than shrinking cards, tables and labels, or
-    // reflowing the drawing.
+    // The scene keeps its columns at any width and fits the stage's: a row's
+    // labels give back the gap they opened before cards narrow. Narrower than
+    // its cards need, it overflows the stage, which asks its host for the
+    // width and is scrolled sideways, rather than shrinking cards, tables and
+    // labels, or reflowing the drawing.
     const layout = () => {
-      sizeGap();
+      grid.style.columnGap = "";
+      grid.style.minWidth = "";
       const style = getComputedStyle(grid);
+      const floor = parseFloat(style.columnGap);
 
       const padding =
         parseFloat(style.paddingLeft) + parseFloat(style.paddingRight);
 
-      const gap = parseFloat(style.columnGap);
+      sizeGap();
+      const wanted = parseFloat(getComputedStyle(grid).columnGap);
       const effective = Math.ceil(columns / unitSpan());
-      grid.style.minWidth = `${effective * 140 + (effective - 1) * gap + padding}px`;
+      const room = grid.clientWidth - padding;
+
+      if (effective > 1 && room - (effective - 1) * wanted < effective * 140)
+        grid.style.columnGap = `${Math.max(floor, (room - effective * 140) / (effective - 1))}px`;
+
+      grid.style.minWidth = `${effective * 140 + (effective - 1) * floor + padding}px`;
     };
 
     let layoutFrame: number | undefined;
@@ -2771,7 +2839,10 @@ export function stageRuntime(port: MessagePort): void {
       layoutFrame = requestAnimationFrame(runLayout);
     };
 
-    new ResizeObserver(() => wake()).observe(grid);
+    new ResizeObserver(() => {
+      wake();
+      scheduleLayout();
+    }).observe(grid);
     layout();
 
     const nodes = new Map<string, HTMLElement>();
diff --git a/packages/review/app/src/blocks.browser.test.tsx b/packages/review/app/src/blocks.browser.test.tsx
index 9f8ef7713..bfaa77ad0 100644
--- a/packages/review/app/src/blocks.browser.test.tsx
+++ b/packages/review/app/src/blocks.browser.test.tsx
@@ -20,7 +20,11 @@ import { createRoot } from "react-dom/client";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { page } from "vitest/browser";
 
-import { STAGE_MAX_HEIGHT, STAGE_MAX_SLOTS } from "./animation/stage-protocol";
+import {
+  STAGE_MAX_HEIGHT,
+  STAGE_MAX_SLOTS,
+  STAGE_MIN_WIDTH,
+} from "./animation/stage-protocol";
 import type { SavedMap } from "./api-document";
 import {
   BlockErrorBoundary,
@@ -1307,7 +1311,81 @@ describe("block components", () => {
     expect(stages.count("ready")).toBe(1);
   });
 
-  it("keeps a scene's columns at a minimum width, scrolling the block sideways, and opens columns to fit a row's link labels", async () => {
+  it("fits a scene to the block, opening columns to fit a row's link labels as far as the width allows", async () => {
+    const { container } = await mountFixture("animation", {
+      document: [
+        {
+          id: "block-1",
+          type: "animation",
+          title: "Scene",
+          height: "auto",
+          frames: [{ key: "only", title: "Only" }],
+          code: {},
+          // Three cards in a row, each link label as wide as the widest gap
+          // a label opens. The stage reports its columns, gap and whether
+          // every card sits inside it through the error line.
+          html:
+            "<script>const s = whiteboard.scene({ columns: 3 });" +
+            ["a", "b", "c"]
+              .map(
+                (id, i) =>
+                  `s.actor({ id: "${id}", at: [${i + 1}, 1], title: "Sharing host / ${id}" });`,
+              )
+              .join("") +
+            's.link("a", "b", { label: "Existing local board ID, reopened" });' +
+            's.link("b", "c", { label: "Download the published bundle" });' +
+            'setTimeout(() => { const grid = document.querySelector(".wb-scene"); const right = document.documentElement.clientWidth; const inside = [...grid.querySelectorAll(".wb-card")].every((card) => card.getBoundingClientRect().right <= right); const style = getComputedStyle(grid); throw new Error(`columns ${style.gridTemplateColumns.split(" ").length} gap ${parseFloat(style.columnGap) > 48} inside ${inside}`); }, 600);' +
+            "</script>",
+          pins: { repositoryId: "repo", base: "base", head: "head" },
+        },
+      ],
+    });
+
+    expect(await settled(() => alertText(container).includes("columns"))).toBe(
+      true,
+    );
+    expect(alertText(container)).toContain("columns 3 gap true inside true");
+
+    // The stage is no wider than the block, so nothing scrolls.
+    const iframe = container.querySelector("iframe")!;
+    const figure = container.querySelector<HTMLElement>(".animation-block")!;
+    expect(iframe.getBoundingClientRect().width).toBeLessThanOrEqual(
+      figure.clientWidth,
+    );
+  });
+
+  it("keeps a link's label off the parallel line drawn back between the same cards", async () => {
+    const { container } = await mountFixture("animation", {
+      document: [
+        {
+          id: "block-1",
+          type: "animation",
+          title: "Round trip",
+          height: "auto",
+          frames: [{ key: "only", title: "Only" }],
+          code: {},
+          // The stage reports whether any point of the return line falls
+          // inside the outbound label through the error line.
+          html:
+            "<script>const s = whiteboard.scene({ columns: 2 });" +
+            's.actor({ id: "a", at: [1, 1], title: "Desktop" });' +
+            's.actor({ id: "b", at: [2, 1], title: "Host" });' +
+            's.link("a", "b", { label: "Import link" });' +
+            's.link("b", "a");' +
+            'setTimeout(() => { const label = [...document.querySelectorAll(".wb-label")].find((text) => text.textContent === "Import link"); const box = label.getBoundingClientRect(); const back = [...document.querySelectorAll(".wb-arrow")].find((arrow) => !arrow.contains(label)).querySelector(".wb-line"); const length = back.getTotalLength(); let crossed = false; for (let at = 0; at <= length; at += 2) { const p = back.getPointAtLength(at); if (p.x >= box.left && p.x <= box.right && p.y >= box.top && p.y <= box.bottom) crossed = true; } throw new Error(`label crosses ${crossed}`); }, 600);' +
+            "</script>",
+          pins: { repositoryId: "repo", base: "base", head: "head" },
+        },
+      ],
+    });
+
+    expect(await settled(() => alertText(container).includes("label"))).toBe(
+      true,
+    );
+    expect(alertText(container)).toContain("label crosses false");
+  });
+
+  it("keeps a scene's columns in a block narrower than its cards need, scrolling it sideways", async () => {
     const { container } = await mountFixture("animation", {
       document: [
         {
@@ -1317,10 +1395,9 @@ describe("block components", () => {
           height: "auto",
           frames: [{ key: "only", title: "Only" }],
           code: { status: "head/order.ts#L1" },
-          // Six cards in a row with a label that opens the gap, and a wide
-          // peek: wider than any block, so the stage asks for its width. Once
-          // the stage is as wide as the scene, or time is up, it reports its
-          // columns, gap and card widths through the error line.
+          // Six cards in a row and a wide peek: wider than any block, so the stage asks for its width. Once the stage
+          // is as wide as the scene, or time is up, it reports its columns
+          // and card widths through the error line.
           html:
             "<script>const s = whiteboard.scene({ columns: 6 });" +
             ["a", "b", "c", "d", "e", "f"]
@@ -1332,7 +1409,7 @@ describe("block components", () => {
             's.link("a", "b", { label: "init + port to the stage" });' +
             's.peek("status", { at: [1, 2, 1, 6] });' +
             "const deadline = Date.now() + 3000;" +
-            'const check = () => { const grid = document.querySelector(".wb-scene"); const fits = grid.getBoundingClientRect().width <= document.documentElement.clientWidth; if (!fits && Date.now() < deadline) { setTimeout(check, 50); return; } const style = getComputedStyle(grid); const widths = [...grid.querySelectorAll(".wb-card")].map((card) => card.getBoundingClientRect().width); throw new Error(`columns ${style.gridTemplateColumns.split(" ").length} gap ${parseFloat(style.columnGap) > 48} cards ${Math.min(...widths) >= 140} fits ${fits}`); };' +
+            'const check = () => { const grid = document.querySelector(".wb-scene"); const fits = grid.getBoundingClientRect().width <= document.documentElement.clientWidth; if (!fits && Date.now() < deadline) { setTimeout(check, 50); return; } const style = getComputedStyle(grid); const widths = [...grid.querySelectorAll(".wb-card")].map((card) => card.getBoundingClientRect().width); throw new Error(`columns ${style.gridTemplateColumns.split(" ").length} cards ${Math.min(...widths) >= 140} fits ${fits}`); };' +
             "setTimeout(check, 300);" +
             "</script>",
           pins: { repositoryId: "repo", base: "base", head: "head" },
@@ -1343,9 +1420,7 @@ describe("block components", () => {
     expect(await settled(() => alertText(container).includes("columns"))).toBe(
       true,
     );
-    expect(alertText(container)).toContain(
-      "columns 6 gap true cards true fits true",
-    );
+    expect(alertText(container)).toContain("columns 6 cards true fits true");
 
     // The block scrolls the stage, at least six cards and their gaps wide.
     const iframe = container.querySelector("iframe")!;
@@ -1392,6 +1467,37 @@ describe("block components", () => {
     ).toBe("auto");
   });
 
+  it("lays a stage out at the board's reading width in a narrower block, scrolling it sideways", async () => {
+    const { container } = await mountFixture(
+      "animation",
+      {
+        document: documentSchema.parse([
+          {
+            id: "block-1",
+            type: "animation",
+            title: "Small",
+            height: 160,
+            frames: [{ key: "a", title: "One" }],
+            code: {},
+            html: '<div class="wb-row">Small</div>',
+            pins: { repositoryId: "repo", base: "base", head: "head" },
+          },
+        ]),
+      },
+      "width: 480px",
+    );
+
+    const iframe = (await settled(() => container.querySelector("iframe")))!;
+    const figure = container.querySelector<HTMLElement>(".animation-block")!;
+    expect(figure.clientWidth).toBeLessThan(STAGE_MIN_WIDTH);
+    expect(iframe.getBoundingClientRect().width).toBeGreaterThanOrEqual(
+      STAGE_MIN_WIDTH,
+    );
+    expect(
+      getComputedStyle(iframe.parentElement!.parentElement!).overflowX,
+    ).toBe("auto");
+  });
+
   it("starts on the first scenario and switches the stage's path from the picker", async () => {
     const { container } = await mountFixture("animation", {
       document: [
diff --git a/packages/review/instructions/animation.md b/packages/review/instructions/animation.md
index 39b596bd0..76b60afaa 100644
--- a/packages/review/instructions/animation.md
+++ b/packages/review/instructions/animation.md
@@ -18,6 +18,8 @@ For a new animation, write the same files into a folder of your own and publish
 
 Before you write HTML, read the two files whose paths `session_checkout` and these instructions give: `whiteboard.css`, the classes and `--wb-*` variables every animation loads, and `whiteboard.d.ts`, the `whiteboard` API. Use the classes and variables rather than your own colors, sizes or fonts, so the animation matches the board in light and dark.
 
+Draw for a stage 720px wide: the width of the board's reading column, and the narrowest the stage gets. Fit that width with nothing past its right edge, so the reader never scrolls sideways on a full-size board: a scene fits three columns of cards, with room for link labels between them. A wider board or fullscreen gives the stage more room, and a narrower window keeps it at 720px and scrolls it sideways, so do not shrink the drawing for small widths. A scene keeps its columns at any width.
+
 Start from the reference closest to what you are drawing, listed under References at the end of these instructions: copy its folder into your draft, then change the data, the prose and the code links.
 
 There are three ways to draw.
@@ -39,7 +41,7 @@ For a few services, processes or stores with messages moving between them, use `
 </script>
 ```
 
-A scene names frame keys, so a new scenario means new frames and a few new lines like these. While a frame shows, each message it sends is the label of its edge and the edge takes the marker colour, however the reader reached the frame; stepping into the frame rides a chip from the sender that settles into the label. A card's `id` is its element's id, so your own CSS can address it as `#api`. A scene keeps its columns at any width: when the board is narrower than its cards need, the block scrolls the drawing sideways rather than reflowing it.
+A scene names frame keys, so a new scenario means new frames and a few new lines like these. While a frame shows, each message it sends is the label of its edge and the edge takes the marker colour, however the reader reached the frame; stepping into the frame rides a chip from the sender that settles into the label. A card's `id` is its element's id, so your own CSS can address it as `#api`.
 
 Give each card the silhouette of what it stands for, so a reader tells the parts apart before reading a title: `form: "window"` for a client, `"bars"` for a service or worker of this codebase, `"cylinder"` for a database, `"drum"` for a cache, `"stack"` for a queue or topic, `"pill"` for a start or end, `"cut"` for a branch. A third-party service or API gets `outline: "dashed"`. `color` sets a border colour at rest (`teal` for stores, `violet` for queues, `accent` for the part under discussion); a frame's `tone` takes over while set. The same `data-form`, `data-outline` and `data-color` work on a `wb-node` in a hand-made layout.