diff --git a/surfaces/shell/index.html b/surfaces/shell/index.html
index aa2e417..7bfdf6a 100644
--- a/surfaces/shell/index.html
+++ b/surfaces/shell/index.html
@@ -1,11 +1,11 @@
 <!doctype html>
-<html lang="en">
+<html lang="en" style="min-width: 0; scrollbar-gutter: stable">
   <head>
     <meta charset="utf-8" />
     <meta content="width=device-width, initial-scale=1" name="viewport" />
     <title>Ekklesia</title>
   </head>
-  <body>
+  <body style="margin: 0; min-width: 0">
     <!-- The shell's root. A module UI never receives this element: it is given a container the
          shell creates inside its own layout. -->
     <div id="ekklesia-host"></div>
diff --git a/surfaces/shell/src/browser/HostApp.tsx b/surfaces/shell/src/browser/HostApp.tsx
index dbae698..c2ad849 100644
--- a/surfaces/shell/src/browser/HostApp.tsx
+++ b/surfaces/shell/src/browser/HostApp.tsx
@@ -115,8 +115,10 @@ const shellStyles = {
   actions: { display: "flex", gap: designToken("spacing.sm"), justifyContent: "center" },
   aside: {
     borderRight: `${designToken("border.width")} solid ${designToken("border.divider")}`,
+    boxSizing: "border-box",
     minWidth: "13rem",
     padding: designToken("spacing.lg"),
+    width: "13rem",
   },
   groupLabel: {
     color: designToken("text.muted"),
@@ -140,10 +142,17 @@ const shellStyles = {
     color: designToken("text.primary"),
     display: "grid",
     fontFamily: designToken("typography.family-sans"),
-    gridTemplateColumns: "auto 1fr",
+    gridTemplateColumns: "13rem minmax(0, 1fr)",
     minHeight: "100vh",
+    minWidth: 0,
+    width: "100%",
+  },
+  main: {
+    boxSizing: "border-box",
+    minWidth: 0,
+    overflowX: "auto",
+    padding: designToken("spacing.lg"),
   },
-  main: { padding: designToken("spacing.lg") },
   message: {
     color: designToken("text.secondary"),
     display: "grid",
@@ -531,7 +540,7 @@ export function HostApp(props: HostAppProps): ReactElement {
   const { groups, ungrouped } = bootstrap.navigation;
 
   return (
-    <div style={shellStyles.layout}>
+    <div data-testid="host-shell" style={shellStyles.layout}>
       <header style={shellStyles.header}>
         <h1 style={shellStyles.heading}>Ekklesia</h1>
         <Button data-testid="host-logout" onClick={() => void signOut()}>
diff --git a/surfaces/shell/src/browser/ModuleUiFrame.tsx b/surfaces/shell/src/browser/ModuleUiFrame.tsx
index 3344472..3957769 100644
--- a/surfaces/shell/src/browser/ModuleUiFrame.tsx
+++ b/surfaces/shell/src/browser/ModuleUiFrame.tsx
@@ -116,7 +116,10 @@ export function ModuleUiFrame(props: ModuleUiFrameProps): ReactElement {
   }, [config, descriptor, importModule, onAuthorizationResponse, principal]);
 
   return (
-    <section aria-live="polite" style={{ display: "grid", gap: "0.75rem" }}>
+    <section
+      aria-live="polite"
+      style={{ display: "grid", gap: "0.75rem", minWidth: 0, width: "100%" }}
+    >
       {status === "loading" ? (
         <p data-testid="module-loading" style={{ color: "#57534e", margin: 0 }}>
           Loading module…
@@ -128,7 +131,11 @@ export function ModuleUiFrame(props: ModuleUiFrameProps): ReactElement {
           to check the installation.
         </p>
       ) : undefined}
-      <div data-testid="module-container" ref={containerRef} />
+      <div
+        data-testid="module-container"
+        ref={containerRef}
+        style={{ minWidth: 0, width: "100%" }}
+      />
     </section>
   );
 }
diff --git a/surfaces/shell/tests/host-app.test.tsx b/surfaces/shell/tests/host-app.test.tsx
index 82afa96..e53e0c6 100644
--- a/surfaces/shell/tests/host-app.test.tsx
+++ b/surfaces/shell/tests/host-app.test.tsx
@@ -971,6 +971,20 @@ describe("Ekklesia shell — Dashboard navigation", () => {
 });
 
 describe("Ekklesia shell — mount lifecycle", () => {
+  it("keeps fixed shell columns while module content changes width", async () => {
+    await render({ apiBaseUrl, fetch: platformFetch() }, async () => artifact("example", "0.1.0"));
+
+    const shell = query("host-shell") as HTMLElement;
+    const aside = shell.querySelector("aside") as HTMLElement;
+    const main = shell.querySelector("main") as HTMLElement;
+    const frame = query("module-container") as HTMLElement;
+    expect(shell.style.gridTemplateColumns).toBe("13rem minmax(0, 1fr)");
+    expect(aside.style.width).toBe("13rem");
+    expect(main.style.minWidth).toBe("0");
+    expect(main.style.overflowX).toBe("auto");
+    expect(frame.style.minWidth).toBe("0");
+  });
+
   it("mounts the first module into the host's own container", async () => {
     const mountedInto: Element[] = [];
     await render({ apiBaseUrl, fetch: platformFetch() }, async () =>