diff --git a/src/client/sidebar.module.css b/src/client/sidebar.module.css
index f65ec26..4220a91 100644
--- a/src/client/sidebar.module.css
+++ b/src/client/sidebar.module.css
@@ -37,6 +37,15 @@
   inset: 0;
   z-index: 25;
   pointer-events: none;
+  /* Clip at the viewport edge: collapsed panels slide off-screen with
+     `transform: translate(102%)`, and a transformed element still
+     contributes to its ancestors' scrollable overflow — without this the
+     hidden panels extended the document's scrollable area, making the whole
+     page draggable left-right and up-down while any panel was collapsed.
+     The host is exactly viewport-sized (inset: 0), so the clip is invisible
+     until a panel slides in from the edge; internal scroll containers and
+     the app's fixed overlay stack are unaffected. */
+  overflow: hidden;
 }
 
 :global([data-dsh-panel-host][data-dsh-panel-host-degraded]) {
diff --git a/tests/e2e/mount.e2e.ts b/tests/e2e/mount.e2e.ts
index 8bf81e8..52a075b 100644
--- a/tests/e2e/mount.e2e.ts
+++ b/tests/e2e/mount.e2e.ts
@@ -191,6 +191,26 @@ test('plugin mounts into the DSH shell and survives a built-in tab sweep', async
   const tabBar = sidebar.locator('[title]')
   await expect(tabBar.first()).toBeAttached({ timeout: 90_000 })
 
+  // Regression: with the panels still COLLAPSED, the document must not grow
+  // beyond the viewport. Collapsed panels are slid off-screen with
+  // `transform: translate(102%)`, and a transformed element still
+  // contributes to its ancestors' scrollable overflow — unclipped, the
+  // hidden panels extended the document's scroll area and the whole page
+  // became draggable left-right and up-down. The panel host clips at the
+  // viewport edge (overflow: hidden), so scrollWidth/scrollHeight must stay
+  // within the viewport here. (Assert <= not == : a classic scrollbar
+  // narrows clientWidth, so scrollWidth may sit slightly under innerWidth.)
+  await expect
+    .poll(
+      () =>
+        page.evaluate(() => ({
+          overflowX: document.documentElement.scrollWidth <= window.innerWidth,
+          overflowY: document.documentElement.scrollHeight <= window.innerHeight,
+        })),
+      { timeout: 30_000 },
+    )
+    .toEqual({ overflowX: true, overflowY: true })
+
   // openByDefault defaults OFF: a fresh session's panel starts collapsed.
   // Expand it through the toggle cluster before the layout push can apply.
   const expandButton = sidebar.getByRole('button', { name: 'Expand sidebar' })