diff --git a/src/app/components/message/Reply.css.ts b/src/app/components/message/Reply.css.ts
index 7e5540fd..33af41a9 100644
--- a/src/app/components/message/Reply.css.ts
+++ b/src/app/components/message/Reply.css.ts
@@ -31,6 +31,7 @@ export const Reply = style({
 });
 
 export const ReplyContent = style({
+  minWidth: 0,
   opacity: config.opacity.P300,
 
   selectors: {
diff --git a/src/app/components/message/layout/Bubble.tsx b/src/app/components/message/layout/Bubble.tsx
index 066a827f..d5293d2e 100644
--- a/src/app/components/message/layout/Bubble.tsx
+++ b/src/app/components/message/layout/Bubble.tsx
@@ -67,7 +67,11 @@ export const BubbleLayout = as<'div', BubbleLayoutProps>(
         <Box
           grow={own ? 'No' : 'Yes'}
           direction="Column"
-          className={classNames(poseRow && css.BubbleColumnPose, own && css.BubbleColumnOwn)}
+          className={classNames(
+            css.BubbleColumn,
+            poseRow && css.BubbleColumnPose,
+            own && css.BubbleColumnOwn
+          )}
         >
           {header && (
             <div
@@ -83,7 +87,7 @@ export const BubbleLayout = as<'div', BubbleLayoutProps>(
           {hideBubble ? (
             children
           ) : (
-            <Box>
+            <Box className={css.BubbleContentRow}>
               <Box
                 className={classNames(
                   css.BubbleContent,
diff --git a/src/app/components/message/layout/layout.css.ts b/src/app/components/message/layout/layout.css.ts
index 9c6b6635..4646ddba 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 c96a4ccf..baad9fc8 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 = {
@@ -113,6 +115,14 @@ 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 33a51b16..656d14c9 100644
--- a/src/preview-demo/server.ts
+++ b/src/preview-demo/server.ts
@@ -136,21 +136,27 @@ 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,
       });
     });
   });