diff --git a/.changeset/layer-dialog-native-scrollbar.md b/.changeset/layer-dialog-native-scrollbar.md
new file mode 100644
index 000000000..9f7e38ae3
--- /dev/null
+++ b/.changeset/layer-dialog-native-scrollbar.md
@@ -0,0 +1,5 @@
+---
+"@cloudflare/kumo": patch
+---
+
+Hide the native scrollbars of the LayerDialog body and Sidebar content with Kumo's own CSS. Base UI only hides them with a style element it injects into the document head, so client routers that swap the head (such as Astro's ClientRouter) or a Content Security Policy that blocks inline styles could leave empty scrollbar tracks along the dialog body.
diff --git a/packages/kumo/src/components/layer-dialog/layer-dialog.test.tsx b/packages/kumo/src/components/layer-dialog/layer-dialog.test.tsx
index 6fde783f1..8cb96a285 100644
--- a/packages/kumo/src/components/layer-dialog/layer-dialog.test.tsx
+++ b/packages/kumo/src/components/layer-dialog/layer-dialog.test.tsx
@@ -538,6 +538,20 @@ describe("LayerDialog.Body edge clipping", () => {
 
     cleanup();
   });
+
+  it("hides native scrollbars without Base UI's injected style", () => {
+    // Base UI forces `overflow: scroll` on the viewport and hides the native
+    // scrollbars only through a <style> it adds to <head> once per document.
+    // Head-swapping client routers (Astro's ClientRouter) remove that style,
+    // which shows empty scrollbar tracks wherever scrollbars are always
+    // visible, so the viewport must carry Kumo's own rule.
+    const { viewport, cleanup } = renderBodyWithLeadingInput(true);
+
+    expect(viewport.style.overflow).toBe("scroll");
+    expect(viewport.className).toMatch(/(^|\s)no-scrollbar(\s|$)/);
+
+    cleanup();
+  });
 });
 
 describe("LayerDialog dismissal", () => {
diff --git a/packages/kumo/src/components/layer-dialog/layer-dialog.tsx b/packages/kumo/src/components/layer-dialog/layer-dialog.tsx
index facb9ac77..480805fc9 100644
--- a/packages/kumo/src/components/layer-dialog/layer-dialog.tsx
+++ b/packages/kumo/src/components/layer-dialog/layer-dialog.tsx
@@ -455,9 +455,14 @@ function LayerDialogBody({ children }: LayerDialogBodyProps) {
       <ScrollAreaBase.Root className="relative flex min-h-0 flex-1 flex-col">
         {/* The top fade falls back to 0px: the body always opens scrolled to
             the top, and Safari skips Base UI's registered 0px initial value,
-            so a non-zero fallback fades out the first row until measured. */}
+            so a non-zero fallback fades out the first row until measured.
+
+            no-scrollbar keeps the native scrollbars of Base UI's
+            `overflow: scroll` viewport hidden. Base UI hides them only with a
+            <style> it adds to <head> once per document, which head-swapping
+            client routers and strict CSPs can drop. */}
         <ScrollAreaBase.Viewport
-          className="min-h-0 flex-1 overscroll-none [mask-image:linear-gradient(to_bottom,transparent_0,black_min(24px,var(--scroll-area-overflow-y-start,0px)),black_calc(100%-min(24px,var(--scroll-area-overflow-y-end,24px))),transparent_100%)]"
+          className="no-scrollbar min-h-0 flex-1 overscroll-none [mask-image:linear-gradient(to_bottom,transparent_0,black_min(24px,var(--scroll-area-overflow-y-start,0px)),black_calc(100%-min(24px,var(--scroll-area-overflow-y-end,24px))),transparent_100%)]"
           onScroll={handleScroll}
         >
           {/* pt-0.5 keeps a first-child control's ring-drawn border and
diff --git a/packages/kumo/src/components/sidebar/sidebar.test.tsx b/packages/kumo/src/components/sidebar/sidebar.test.tsx
index 1f0d36348..61a3c878c 100644
--- a/packages/kumo/src/components/sidebar/sidebar.test.tsx
+++ b/packages/kumo/src/components/sidebar/sidebar.test.tsx
@@ -1164,6 +1164,19 @@ describe("Sidebar contained mode", () => {
   });
 });
 
+describe("Sidebar.Content scrollbar", () => {
+  it("should hide the native scrollbar without Base UI's injected style", () => {
+    render(
+      <TestSidebar defaultOpen>
+        <SidebarContent>Content</SidebarContent>
+      </TestSidebar>,
+    );
+    // Base UI's own rule lives in a <style> that head-swapping client
+    // routers can remove, so the viewport must carry Kumo's.
+    expect(getViewport().className).toMatch(/(^|\s)no-scrollbar(\s|$)/);
+  });
+});
+
 // ============================================================================
 // Mobile behavior
 // ============================================================================
diff --git a/packages/kumo/src/components/sidebar/sidebar.tsx b/packages/kumo/src/components/sidebar/sidebar.tsx
index 4d56eaa9a..93e301877 100644
--- a/packages/kumo/src/components/sidebar/sidebar.tsx
+++ b/packages/kumo/src/components/sidebar/sidebar.tsx
@@ -1063,6 +1063,10 @@ const SidebarContent = forwardRef<
           "h-full px-[11px] py-3 group-not-data-[state=collapsed]/sidebar:px-3.5",
           "transition-[padding] duration-(--sidebar-animation-duration)",
           "overflow-x-hidden!",
+          // Hide the native scrollbar with Kumo's CSS rather than relying on
+          // the <style> Base UI adds to <head>, which head-swapping client
+          // routers and strict CSPs can drop.
+          "no-scrollbar",
           // Scroll fade via CSS mask driven by Base UI overflow CSS variables
           "[mask-image:linear-gradient(to_bottom,transparent_0,black_min(24px,var(--scroll-area-overflow-y-start,24px)),black_calc(100%-min(24px,var(--scroll-area-overflow-y-end,24px))),transparent_100%)]",
         )}