diff --git a/packages/web/e2e/layout.spec.mjs b/packages/web/e2e/layout.spec.mjs
index 0ca0df8..56a329a 100644
--- a/packages/web/e2e/layout.spec.mjs
+++ b/packages/web/e2e/layout.spec.mjs
@@ -15,6 +15,10 @@
  *   ~34px off-screen left, the skills menu ~92px off-screen right — with its autofocused
  *   search box then horizontally scrolling the whole draft page — and the workspace menu
  *   ~143px off-screen right when the ownership pills share one row);
+ * - no page grows the **document**: the app shell is height-constrained and each page scrolls
+ *   inside its own container, so a second scrollbar means an absolutely positioned descendant
+ *   escaped its scroller (the Traces tree and the Agent settings page both had one, visible
+ *   only with a second Agent below a long list);
  * - the sidebar's "New chat" button has no background fill (same gray-scale style as nav items);
  * - the collapsed rail shows, in product-specified order, last conversation / new chat /
  *   Agents / Skills / Models / Costs / Traces / Benchmark with localized (en + zh) hover
@@ -646,6 +650,78 @@ test("layout: mobile chat dropdowns stay inside the viewport", async ({ page })
   ).toBeLessThanOrEqual(40);
 });
 
+/**
+ * The app shell is height-constrained: every page scrolls inside its own container and the
+ * document itself must never scroll. What breaks that is an absolutely positioned descendant
+ * whose containing block is the initial containing block — nothing between it and the root
+ * clips it, so its static position past the fold grows the **document**, producing a second
+ * scrollbar that drags the whole shell up. The Traces tree and the Agent settings page both
+ * had one (the `sr-only` file inputs of their import controls), visible only once a second
+ * Agent sat below a long list. This sweep is the general guard: with the data that exposes it,
+ * no page may grow the document.
+ */
+test("layout: no page grows the document (absolute descendants stay in their scroller)", async ({
+  page,
+}) => {
+  await provisionAndLogin(page.request, "layoutheight", P);
+  const projects = await (await page.request.get(`${BASE}/api/projects`)).json();
+  const projectId = projects.projects[0].projectId;
+  const put = await page.request.put(`${BASE}/api/projects/${projectId}/models`, {
+    data: {
+      defaultModel: { provider: "custom", modelId: "claude-4-8" },
+      models: [
+        {
+          provider: "custom",
+          modelId: "claude-4-8",
+          apiKey: "sk-mock",
+          baseUrl: MOCK,
+          contextWindow: 200000,
+        },
+      ],
+    },
+  });
+  expect(put.ok(), "put models").toBeTruthy();
+  // A second Agent is what moves an import control below the fold; the Traces tree only lists
+  // Sessions that have a Trace file, so each one has to actually run a Task.
+  const second = await page.request.post(`${BASE}/api/projects/${projectId}/agents`, {
+    data: { agentId: "agent_two", name: "Agent Two" },
+  });
+  // 409 = the Agent survives from an earlier run of this spec against the same data root
+  // (provisioning is idempotent by convention here, so a spec can be rerun on its own).
+  expect([201, 409], "create second agent").toContain(second.status());
+  for (const agentId of ["default_agent", "agent_two"]) {
+    for (let i = 0; i < (agentId === "default_agent" ? 14 : 2); i += 1) {
+      const created = await (
+        await page.request.post(`${BASE}/api/projects/${projectId}/agents/${agentId}/sessions`, {
+          data: { provider: "custom", modelId: "claude-4-8" },
+        })
+      ).json();
+      await page.request.post(`${BASE}/api/sessions/${created.session.sessionId}/tasks`, {
+        data: { input: [{ type: "text", text: "hi" }] },
+      });
+    }
+  }
+
+  // A short viewport puts the second Agent's node below the fold with a handful of Sessions
+  // instead of dozens — the same geometry a full-height window reaches with a longer list.
+  // Not shorter than 420: below ~412px the sidebar's own fixed chrome (project switcher, the
+  // eight nav entries, the user row) no longer fits, which grows the document for its own
+  // unrelated reason and would mask what this test is after.
+  await page.setViewportSize({ width: 1440, height: 420 });
+  const paths = ["/traces", "/chat", "/agents", "/agents/default_agent", "/skills", "/models"];
+  const overflowing = [];
+  for (const path of paths) {
+    await page.goto(`${BASE}${path}`);
+    await page.waitForTimeout(1200);
+    const grew = await page.evaluate(() => {
+      const de = document.documentElement;
+      return de.scrollHeight - de.clientHeight;
+    });
+    if (grew > 0) overflowing.push(`${path} (+${grew}px)`);
+  }
+  expect(overflowing, "pages whose document scrolls").toEqual([]);
+});
+
 test("layout: login — blank start, non-crossing traces, lang/theme controls", async ({ page }) => {
   await page.goto(`${BASE}/login`);
   await page.getByRole("heading", { name: "PenguinHarness" }).waitFor();
diff --git a/packages/web/src/components/ui/hidden-file-input.tsx b/packages/web/src/components/ui/hidden-file-input.tsx
new file mode 100644
index 0000000..00dd44a
--- /dev/null
+++ b/packages/web/src/components/ui/hidden-file-input.tsx
@@ -0,0 +1,25 @@
+/**
+ * File input that is visually hidden but still Tab-focusable — the picker every "upload" /
+ * "import" control in the app wraps in its own `<label>`.
+ *
+ * `display: none` would drop it out of the focus order, so the box has to be visually hidden
+ * the `sr-only` way, which means `position: absolute`. That is exactly the shape that used to
+ * escape its scroller and stretch the document (see the scroll-container invariant in
+ * styles.css): dropped into a `static` ancestor chain, its containing block became the
+ * initial containing block, so a control sitting past the fold added document-level
+ * scrollable overflow and a second scrollbar.
+ *
+ * Hence the wrapper: it is `relative`, so the containing block travels **with the control**
+ * instead of depending on whichever ancestor happens to be positioned. The wrapper is
+ * `contents`-free on purpose — it is a real box, but an empty inline one with an absolutely
+ * positioned child, so it contributes no width and no height to the label around it.
+ */
+import type { InputHTMLAttributes } from "react";
+
+export function HiddenFileInput(props: Omit<InputHTMLAttributes<HTMLInputElement>, "type">) {
+  return (
+    <span className="relative">
+      <input type="file" className="sr-only" {...props} />
+    </span>
+  );
+}
diff --git a/packages/web/src/features/agents/agent-settings-page.tsx b/packages/web/src/features/agents/agent-settings-page.tsx
index 566f81d..0503313 100644
--- a/packages/web/src/features/agents/agent-settings-page.tsx
+++ b/packages/web/src/features/agents/agent-settings-page.tsx
@@ -27,6 +27,7 @@ import { useProject } from "../../state/project";
 import { Tabs } from "../../components/ui/tabs";
 import { Button } from "../../components/ui/button";
 import { toastError, toastInfo, toastSuccess } from "../../components/ui/toast";
+import { HiddenFileInput } from "../../components/ui/hidden-file-input";
 import { Input, Textarea } from "../../components/ui/input";
 import { OptionMenu, type OptionMenuChoice } from "../../components/ui/option-menu";
 import { Switch } from "../../components/ui/switch";
@@ -156,7 +157,9 @@ export function AgentSettingsPage() {
   }
 
   return (
-    <div className="no-scrollbar h-full overflow-y-auto p-4 md:p-6">
+    /* relative: a scroller is its own containing block (see the invariant in styles.css) —
+       this page's snapshot-import control used to grow the document from below the fold. */
+    <div className="no-scrollbar relative h-full overflow-y-auto p-4 md:p-6">
       <div className="mx-auto max-w-3xl">
         <Button
           variant="ghost"
@@ -351,14 +354,7 @@ function OverviewTab({
             <label
               className={`${TRANSFER_BUTTON_CLASS} ${importing ? "pointer-events-none opacity-60" : ""}`}
             >
-              {/* sr-only rather than hidden: keeps it keyboard-Tab-focusable (same as the workspace-browser upload). */}
-              <input
-                type="file"
-                accept=".tar.gz,.tgz"
-                className="sr-only"
-                disabled={importing}
-                onChange={onPickFile}
-              />
+              <HiddenFileInput accept=".tar.gz,.tgz" disabled={importing} onChange={onPickFile} />
               {importing ? S.agent.importing : S.agent.importSnapshot}
             </label>
           )}
diff --git a/packages/web/src/features/chat/workspace-browser.tsx b/packages/web/src/features/chat/workspace-browser.tsx
index b9f82c9..293be82 100644
--- a/packages/web/src/features/chat/workspace-browser.tsx
+++ b/packages/web/src/features/chat/workspace-browser.tsx
@@ -27,6 +27,7 @@ import { Button } from "../../components/ui/button";
 import { ConfirmModal } from "../../components/ui/confirm-modal";
 import { toastError, toastSuccess } from "../../components/ui/toast";
 import { Dropdown } from "../../components/ui/dropdown";
+import { HiddenFileInput } from "../../components/ui/hidden-file-input";
 import { SkeletonList } from "../../components/ui/skeleton";
 import { CodeBlock } from "./code-block";
 
@@ -745,14 +746,7 @@ export function WorkspaceBrowser({
         </Button>
         {/* Matches the same visual style and font size (sm = text-xs) as the adjacent ghost Buttons (Details/Refresh): no border, light background on hover. */}
         <label className="inline-flex cursor-pointer items-center rounded-md border border-transparent bg-transparent px-2.5 py-1 text-xs font-medium text-gray-600 transition-colors duration-150 focus-within:ring-2 focus-within:ring-gray-400/30 hover:bg-gray-100 hover:text-gray-900 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-gray-100">
-          {/* sr-only rather than hidden: keyboard users can still Tab-focus it (display:none would remove it from the focus order). */}
-          <input
-            type="file"
-            multiple
-            className="sr-only"
-            onChange={onUpload}
-            disabled={uploading}
-          />
+          <HiddenFileInput multiple onChange={onUpload} disabled={uploading} />
           {uploading ? S.common.saving : S.files.upload}
         </label>
       </div>
diff --git a/packages/web/src/features/traces/traces-page.tsx b/packages/web/src/features/traces/traces-page.tsx
index ea07f1a..fb70b95 100644
--- a/packages/web/src/features/traces/traces-page.tsx
+++ b/packages/web/src/features/traces/traces-page.tsx
@@ -17,6 +17,7 @@ import { formatBytes } from "../../lib/format";
 import { agentDisplayName, useProject } from "../../state/project";
 import { AgentAvatar } from "../../components/ui/agent-avatar";
 import { Chevron } from "../../components/ui/chevron";
+import { HiddenFileInput } from "../../components/ui/hidden-file-input";
 import { DownloadIcon, UploadIcon } from "../../components/ui/icons";
 import { toastError } from "../../components/ui/toast";
 import { Truncated } from "../../components/ui/truncated";
@@ -197,14 +198,7 @@ function AgentNode({
               importing ? "pointer-events-none opacity-60" : ""
             }`}
           >
-            {/* sr-only rather than hidden: keeps it keyboard-Tab-focusable (same as the snapshot import). */}
-            <input
-              type="file"
-              accept=".jsonl"
-              className="sr-only"
-              disabled={importing}
-              onChange={onPickFile}
-            />
+            <HiddenFileInput accept=".jsonl" disabled={importing} onChange={onPickFile} />
             <UploadIcon size={13} />
             <span className="sr-only">{importing ? S.traces.importing : S.traces.importTrace}</span>
           </label>
@@ -297,8 +291,12 @@ export function TracesPage() {
 
   return (
     <div className="flex h-full flex-col md:flex-row">
-      {/* Directory tree: Agent → Session title (≥md left column; <md top collapsible area) */}
-      <aside className="max-h-52 shrink-0 overflow-y-auto border-b border-gray-200 bg-gray-50 px-1 py-2 md:max-h-none md:w-72 md:border-b-0 md:border-r dark:border-gray-800 dark:bg-gray-900">
+      {/* Directory tree: Agent → Session title (≥md left column; <md top collapsible area).
+          relative: a scroller is its own containing block — the invariant and the failure it
+          prevents are documented in styles.css; this is where it first bit (an Agent node past
+          the fold put its import control's absolutely positioned box at a document-level
+          offset, growing the document). */}
+      <aside className="relative max-h-52 shrink-0 overflow-y-auto border-b border-gray-200 bg-gray-50 px-1 py-2 md:max-h-none md:w-72 md:border-b-0 md:border-r dark:border-gray-800 dark:bg-gray-900">
         <p className="px-3 pb-1 text-xs font-bold uppercase tracking-wide text-gray-500">
           {S.traces.title}
         </p>
diff --git a/packages/web/src/styles.css b/packages/web/src/styles.css
index 18d958d..3f57a0c 100644
--- a/packages/web/src/styles.css
+++ b/packages/web/src/styles.css
@@ -76,6 +76,29 @@
   #root {
     height: 100%;
   }
+  /* Every scroll container is its own containing block.
+     The shell is height-constrained (the three rules above) and each page scrolls inside its
+     own container, so the document must never scroll. What breaks that is an absolutely
+     positioned descendant of a scroller that is itself `static`: its containing block is then
+     the initial containing block, which nothing clips — not the scroller's own overflow, not
+     an `overflow-hidden` ancestor — so its static position past the fold becomes the
+     *document's* scrollable overflow, and a second scrollbar appears that drags the whole
+     shell. That bug shipped three times (sidebar session list, Traces tree, Agent settings),
+     every time through a visually-hidden-but-focusable control, so the invariant is set here
+     once instead of being remembered per scroller.
+     Scoped to the scrollable utilities on purpose: `overflow-hidden` is also used as a plain
+     clipping box (the docked panels' clipping windows), where forcing a containing block is
+     more likely to surprise than to help. Declared in `base`, so an explicit
+     absolute/fixed/sticky utility on the same element still wins; `position: relative` at
+     `z-index: auto` creates no stacking context, so the convention in the file header holds. */
+  .overflow-auto,
+  .overflow-x-auto,
+  .overflow-y-auto,
+  .overflow-scroll,
+  .overflow-x-scroll,
+  .overflow-y-scroll {
+    position: relative;
+  }
   body {
     @apply bg-white text-gray-900 antialiased dark:bg-gray-950 dark:text-gray-100;
     font-family: