diff --git a/src/app/components/message/layout/Bubble.tsx b/src/app/components/message/layout/Bubble.tsx
index bd2056b9..6f4e94b0 100644
--- a/src/app/components/message/layout/Bubble.tsx
+++ b/src/app/components/message/layout/Bubble.tsx
@@ -60,13 +60,13 @@ export const BubbleLayout = as<'div', BubbleLayoutProps>(
         <Box
           grow={own ? 'No' : 'Yes'}
           direction="Column"
-          className={classNames(own && css.BubbleColumnOwn)}
+          className={classNames(css.BubbleColumn, own && css.BubbleColumnOwn)}
         >
           {header && <div className={classNames(own && css.BubbleHeaderOwn)}>{header}</div>}
           {hideBubble ? (
             children
           ) : (
-            <Box>
+            <Box className={css.BubbleContentRow}>
               <Box
                 className={classNames(
                   css.BubbleContent,
diff --git a/src/app/components/message/layout/bubbleBounds.test.ts b/src/app/components/message/layout/bubbleBounds.test.ts
new file mode 100644
index 00000000..4a9ad4da
--- /dev/null
+++ b/src/app/components/message/layout/bubbleBounds.test.ts
@@ -0,0 +1,48 @@
+import { readFileSync } from 'fs';
+import { describe, expect, it } from 'vitest';
+import zlib from 'zlib';
+import { compile, processVanillaFile } from '@vanilla-extract/integration';
+
+// A long reply quote (nowrap) sized the own-message row to 782px on a 390px phone. The row, the column and the bubble
+// must be able to shrink to their parent, so pin the emitted declarations (as phoneTapTargets.test.ts does).
+const emitted = async () => {
+  const filePath = `${process.cwd()}/src/app/components/message/layout/layout.css.ts`;
+  const { source } = await compile({ filePath, cwd: process.cwd(), identOption: 'debug' });
+  const code = await processVanillaFile({ source, filePath, identOption: 'debug' });
+  const bytes = Buffer.from(
+    (code.match(/source=#?([A-Za-z0-9+/=]+)/) as RegExpMatchArray)[1],
+    'base64'
+  );
+  return (bytes[0] === 0x1f && bytes[1] === 0x8b ? zlib.gunzipSync(bytes) : bytes).toString();
+};
+
+const declarations = (css: string, className: string): string[] => {
+  const rule = new RegExp(`^\\.layout_${className}__[\\w-]+ \\{\\n([^}]*)\\}`, 'm').exec(css);
+  expect(rule, `no .layout_${className} rule emitted`).toBeTruthy();
+  return (rule as RegExpExecArray)[1].split('\n').map((l) => l.trim().replace(/;$/, ''));
+};
+
+describe('bubble rows shrink to their column', () => {
+  it.each([
+    ['BubbleColumn', ['min-width: 0']],
+    ['BubbleContentRow', ['min-width: 0', 'max-width: 100%']],
+    ['BubbleContent', ['min-width: 0']],
+  ])('%s', async (name, expected) => {
+    const css = await emitted();
+    expect(declarations(css, name)).toEqual(expect.arrayContaining(expected));
+  });
+
+  it('BubbleContent keeps its 800px cap inside the column width', async () => {
+    const css = await emitted();
+    expect(declarations(css, 'BubbleContent').join('\n')).toMatch(/max-width: min\(100%, 50rem\)/);
+  });
+
+  it('Bubble.tsx puts BubbleColumn on the column and BubbleContentRow on the row', () => {
+    const tsx = readFileSync(
+      `${process.cwd()}/src/app/components/message/layout/Bubble.tsx`,
+      'utf8'
+    );
+    expect(tsx).toMatch(/classNames\(css\.BubbleColumn,/);
+    expect(tsx).toMatch(/<Box className=\{css\.BubbleContentRow\}>/);
+  });
+});
diff --git a/src/app/components/message/layout/layout.css.ts b/src/app/components/message/layout/layout.css.ts
index 3037e66b..3d1a9c83 100644
--- a/src/app/components/message/layout/layout.css.ts
+++ b/src/app/components/message/layout/layout.css.ts
@@ -139,8 +139,22 @@ export const BubbleBefore = style({
   minWidth: toRem(36),
 });
 
+// minWidth 0 down the column: a flex item never shrinks below its content (a reply quote's unbroken text),
+// so without it a long quote pushes the column past the row, and a right-aligned row pushes it off the left edge.
+export const BubbleColumn = style({
+  minWidth: 0,
+});
+
+// The row around a bubble sizes to its content in a right-aligned (own) column, and a quote's nowrap text
+// makes that content as wide as the quote: cap the row at the column so the bubble can shrink inside it.
+export const BubbleContentRow = style({
+  minWidth: 0,
+  maxWidth: '100%',
+});
+
 export const BubbleContent = style({
-  maxWidth: toRem(800),
+  minWidth: 0,
+  maxWidth: `min(100%, ${toRem(800)})`,
   padding: config.space.S200,
   backgroundColor: color.SurfaceVariant.Container,
   color: color.SurfaceVariant.OnContainer,
diff --git a/src/preview-demo/fixtures.ts b/src/preview-demo/fixtures.ts
index 18b2fac1..f88ff746 100644
--- a/src/preview-demo/fixtures.ts
+++ b/src/preview-demo/fixtures.ts
@@ -33,6 +33,8 @@ export type DemoLine = {
   body: string;
   html?: string;
   pose?: string;
+  // Index of an earlier line in the same room this one replies to.
+  replyTo?: number;
   minutesAgo: number;
 };
 export type DemoRoom = {
@@ -116,6 +118,14 @@ export const ROOMS: DemoRoom[] = [
       },
       // A long display name: the name pill has to show all of it on a phone.
       { from: user('heimdall'), body: 'Ok.', pose: 'thinking', minutesAgo: 20 },
+      // A long message to reply to: the quote above each reply must stay inside its column on a phone.
+      {
+        from: user('visitor'),
+        body: "The read exists from 2026-11-04. It's research only, nothing is wired up yet, so please do not treat the notes_in_a_single_unbroken_token_that_keeps_going_past_the_edge as a decision.",
+        minutesAgo: 12,
+      },
+      { from: ME, body: 'Noted, thank you.', replyTo: 8, minutesAgo: 11 },
+      { from: user('void'), body: 'Same here.', replyTo: 8, minutesAgo: 10 },
     ],
   },
   {
diff --git a/src/preview-demo/server.ts b/src/preview-demo/server.ts
index ed8cbf46..4a70c95b 100644
--- a/src/preview-demo/server.ts
+++ b/src/preview-demo/server.ts
@@ -145,21 +145,27 @@ export const createDemoServer = (now: () => number = Date.now): DemoServer => {
 
   ROOMS.forEach((room) => {
     stateOf.set(room.id, buildState(room));
+    const lineIds: string[] = [];
     room.lines.forEach((l) => {
+      const event = mk(
+        'm.room.message',
+        l.from,
+        {
+          msgtype: 'm.text',
+          body: l.body,
+          ...(l.html ? { format: 'org.matrix.custom.html', formatted_body: l.html } : {}),
+          ...(l.pose ? { [POSE_KEY]: l.pose } : {}),
+          ...(l.replyTo === undefined
+            ? {}
+            : { 'm.relates_to': { 'm.in_reply_to': { event_id: lineIds[l.replyTo] } } }),
+        },
+        now() - l.minutesAgo * 60 * 1000
+      );
+      lineIds.push(event.event_id);
       timeline.push({
         version: 1,
         roomId: room.id,
-        event: mk(
-          'm.room.message',
-          l.from,
-          {
-            msgtype: 'm.text',
-            body: l.body,
-            ...(l.html ? { format: 'org.matrix.custom.html', formatted_body: l.html } : {}),
-            ...(l.pose ? { [POSE_KEY]: l.pose } : {}),
-          },
-          now() - l.minutesAgo * 60 * 1000
-        ),
+        event,
       });
     });
   });