diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index 652cda5ca..e266e2cba 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -155,6 +155,9 @@ never the field's only name. Help and validation belong to that field without
 changing the alignment of neighboring controls and their actions.
 
 Short fixed choices keep native selects, including the platform's own popup.
+A control never widens its column: a select shows its chosen label on one
+line, clipped at its own edge, and the full label stays in the platform's
+popup.
 Model selection uses the shared editable chooser: suggestions assist typing
 without becoming an allowlist. A saved unknown model remains editable; a
 catalog refresh preserves the real input, selection and composition. Escape
diff --git a/docs/DEVELOPMENT.md b/docs/DEVELOPMENT.md
index 36bc5455f..849a0da27 100644
--- a/docs/DEVELOPMENT.md
+++ b/docs/DEVELOPMENT.md
@@ -3169,6 +3169,26 @@ SPA, not a relocatable-page or multi-instance panel framework.
   `.style.<property>` assignments are review debt (a dynamic measured value
   may update a narrowly named custom property when that is the real runtime
   data flow).
+- Containment: a control never widens its column, and horizontal overflow
+  lives in the wrapper that owns the wide content and declares
+  `overflow-x: auto` (code block, `.md-table-wrap`, tab strip, Costs table
+  cells) — never in a page scroll body, whose `overflow-y: auto` alone
+  already makes `overflow-x` compute to `auto`. The shared
+  `select.ui-control` recipe therefore clips its own value
+  (`overflow: hidden`): WebKit computes `overflow: visible` on a native
+  select, so an unclipped option label becomes scrollable overflow of that
+  page scroller. A grid track holding controls takes a minimum that yields
+  to its container — `minmax(0, …)`, or
+  `repeat(auto-fit, minmax(min(100%, Npx), 1fr))`; a fixed px minimum
+  rescued only by a viewport media query is review debt, because the
+  viewport does not know how wide the content column is. The global webkit
+  scrollbar recipe sizes both axes. Enforced by
+  `tests/test_web_typography_static.py::test_select_control_clips_its_value`,
+  its `::test_webkit_scrollbar_recipe_covers_both_axes` neighbour, and
+  `tests/test_ui_settings_overflow_browser.py`; two gaps stay open — the
+  wizard document loads `ui.css` without `style.css` and keeps native
+  scrollbars, and an element setting the standard
+  `scrollbar-width`/`scrollbar-color` opts out of the webkit recipe on Blink.
 - One semantic button variant expresses one action role: neutral Settings
   and onboarding controls use the existing `.btn.btn-default`; a one-action
   result row uses the named `.settings-action-row` contract (status first,
@@ -3260,7 +3280,11 @@ stealing usable text space; use the shared responsive component before
 adding a page-specific layout. A visible change is inspected with vision in
 at least one relevant real consumer flow. A stored screenshot alone is not
 verification; mobile or WebKit is not a universal requirement and is
-selected from risk. Review-only: scored by CHECKLISTS items 2(i) and 30
+selected from risk. Containment is the WebKit-sensitive exception — a native
+select is not clipped there — so a change to a control recipe or a page
+scroll body is verified on Playwright WebKit as well as Chromium, measuring
+overflow on the scroll body's `scrollWidth` rather than on
+`documentElement`. Review-only: scored by CHECKLISTS items 2(i) and 30
 (`web_design_system`).
 
 ### Browser dialogs
diff --git a/tests/test_ui_settings_overflow_browser.py b/tests/test_ui_settings_overflow_browser.py
new file mode 100644
index 000000000..4a3c09c49
--- /dev/null
+++ b/tests/test_ui_settings_overflow_browser.py
@@ -0,0 +1,214 @@
+"""WebKit guard: Settings never grows a horizontal scrollbar.
+
+The owner's report was a thick red horizontal scrollbar across the bottom of
+Settings -> Advanced in the packaged desktop shell (WKWebView). Blink's UA
+stylesheet clips a native ``<select>`` (``overflow: clip``); WebKit computes
+``visible``, so a long selected option paints past the control's own box, the
+``.settings-scroll`` page scroller (``overflow-x`` resolves to ``auto``) grows,
+and the scrollbar the app paints for it was UA-thick because the global
+``::-webkit-scrollbar`` recipe sized only ``width``.
+
+Chromium is deliberately NOT exercised here: the class cannot appear there, so
+a second engine would only pin the engine that never had the bug.
+"""
+
+from __future__ import annotations
+
+import os
+import pathlib
+import textwrap
+
+import pytest
+
+pytest_plugins = ("tests.test_ui_smoke_playwright",)
+
+
+# The real trigger from the owner's screenshot: a 65-character sentence as the
+# selected option of an extension settings select.
+LONG_OPTION_LABEL = "Full access (default) — raw owner commands incl. /panic, /restart"
+SECTION_TITLE = "Owner command access"
+
+
+def _write_long_option_settings_extension(data_dir: pathlib.Path) -> str:
+    """Install an exact-hash reviewed extension whose settings form carries the
+    long-label select, rendered by the production settings renderer.
+
+    The field ORDER mirrors the real Telegram section and is load-bearing: the
+    long select is the SECOND field, so it lands in the right-hand column of
+    ``.form-grid.two``. A leak out of a left-column control is absorbed by the
+    empty right half of the grid and never reaches the page scroller, so a
+    left-column reproduction would be green on the unfixed stylesheet too.
+    """
+    from ouroboros.skill_loader import (
+        SkillReviewState,
+        compute_content_hash,
+        save_review_state,
+    )
+
+    name = "settings_overflow_smoke"
+    skill_dir = data_dir / "skills" / "external" / name
+    skill_dir.mkdir(parents=True, exist_ok=True)
+    (skill_dir / "SKILL.md").write_text(
+        textwrap.dedent(
+            f"""\
+            ---
+            name: {name}
+            description: Settings select whose label is long enough to overflow.
+            version: 0.1.0
+            type: extension
+            entry: plugin.py
+            permissions: ["route", "widget"]
+            ---
+            # Settings overflow smoke
+            """
+        ),
+        encoding="utf-8",
+    )
+    plugin = textwrap.dedent(
+        '''\
+        async def save(request):
+            body = await request.json()
+            return {"message": f"Saved {body.get('access') or 'default'} access."}
+
+
+        def register(api):
+            api.register_route("save", save, methods=("POST",))
+            api.register_settings_section(
+                "config",
+                "__SECTION_TITLE__",
+                schema={
+                    "components": [
+                        {
+                            "type": "form",
+                            "id": "access-form",
+                            "route": "save",
+                            "method": "POST",
+                            "submit_label": "Save access",
+                            "fields": [
+                                {
+                                    "name": "language",
+                                    "label": "Language",
+                                    "type": "select",
+                                    "default": "en",
+                                    "options": [
+                                        {"label": "English", "value": "en"},
+                                        {"label": "Russian", "value": "ru"},
+                                    ],
+                                },
+                                {
+                                    "name": "command_mode",
+                                    "label": "Command mode",
+                                    "type": "select",
+                                    "default": "full",
+                                    "options": [
+                                        {"label": "__LONG_LABEL__", "value": "full"},
+                                        {"label": "Strict", "value": "strict"},
+                                    ],
+                                },
+                            ],
+                        }
+                    ]
+                },
+            )
+        '''
+    )
+    plugin = plugin.replace("__LONG_LABEL__", LONG_OPTION_LABEL)
+    plugin = plugin.replace("__SECTION_TITLE__", SECTION_TITLE)
+    (skill_dir / "plugin.py").write_text(plugin, encoding="utf-8")
+    content_hash = compute_content_hash(skill_dir, manifest_entry="plugin.py")
+    save_review_state(
+        data_dir,
+        name,
+        SkillReviewState(status="pass", content_hash=content_hash),
+    )
+    return name
+
+
+@pytest.mark.serial
+@pytest.mark.ui_browser
+def test_settings_advanced_never_scrolls_horizontally_in_webkit(direct_server_with_data):
+    pytest.importorskip("playwright.sync_api", reason="Playwright is not installed")
+    from playwright.sync_api import Error as PlaywrightError
+    from playwright.sync_api import sync_playwright
+
+    url = direct_server_with_data["url"]
+    data_dir = direct_server_with_data["data_dir"]
+    skill = _write_long_option_settings_extension(data_dir)
+    evidence_dir = pathlib.Path(
+        os.environ.get("OUROBOROS_UI_EVIDENCE_DIR", str(data_dir.parent))
+    )
+    evidence_dir.mkdir(parents=True, exist_ok=True)
+    screenshot = evidence_dir / "settings-advanced-webkit-no-horizontal-overflow.png"
+
+    try:
+        with sync_playwright() as pw:
+            browser = pw.webkit.launch(headless=True)
+            # The narrow desktop shell window the owner reported from; the leak
+            # grows as the window narrows, so this is the honest width to pin.
+            page = browser.new_page(viewport={"width": 1000, "height": 680})
+            try:
+                page.goto(url, wait_until="domcontentloaded", timeout=30_000)
+                toggled = page.evaluate(
+                    """async (skill) => {
+                        const response = await fetch(`/api/skills/${encodeURIComponent(skill)}/toggle`, {
+                            method: 'POST',
+                            headers: {'Content-Type': 'application/json'},
+                            body: JSON.stringify({enabled: true}),
+                        });
+                        return {status: response.status, body: await response.json()};
+                    }""",
+                    skill,
+                )
+                assert toggled["status"] == 200, toggled
+                assert toggled["body"].get("enabled") is True, toggled
+
+                page.wait_for_selector('[data-nav-page="settings"]', timeout=30_000)
+                page.click('[data-nav-page="settings"]')
+                page.locator('[data-settings-tab="advanced"]').click()
+                section = page.locator('.settings-extension-section').filter(
+                    has_text=SECTION_TITLE
+                )
+                section.wait_for(state="visible", timeout=30_000)
+                select = section.locator('select[name="command_mode"]')
+                select.wait_for(state="visible", timeout=30_000)
+                # The long option must actually be the selected one, or the
+                # measurement below would prove nothing.
+                assert select.input_value() == "full"
+                assert LONG_OPTION_LABEL in select.inner_text()
+
+                overflow = page.evaluate(
+                    """() => {
+                        const s = document.querySelector('.settings-scroll');
+                        return s.scrollWidth - s.clientWidth;
+                    }"""
+                )
+                widths = select.evaluate(
+                    """(element) => [
+                        element.getBoundingClientRect().width,
+                        element.parentElement.getBoundingClientRect().width,
+                    ]"""
+                )
+                # `.settings-scroll` is an inner scroller, so a full-page shot
+                # would frame the top of Advanced and not the offending control.
+                select.scroll_into_view_if_needed()
+                page.wait_for_timeout(150)
+                # Captured before the assertions so a red run leaves evidence too.
+                page.screenshot(path=str(screenshot), full_page=True)
+
+                assert overflow <= 1, (
+                    "Settings scrolls horizontally: .settings-scroll is "
+                    f"{overflow}px wider than its own box. A native select's "
+                    "value is not clipped by WebKit, so a long option leaks "
+                    "into the page scroller and the app paints the horizontal "
+                    "bar the owner reported."
+                )
+                assert widths[0] <= widths[1] + 1, (
+                    "the select is wider than its grid cell: "
+                    f"{widths[0]}px control in a {widths[1]}px cell"
+                )
+            finally:
+                browser.close()
+    except PlaywrightError as exc:
+        if "Executable doesn't exist" in str(exc) or "playwright install" in str(exc).lower():
+            pytest.skip(str(exc))
+        raise
diff --git a/tests/test_web_typography_static.py b/tests/test_web_typography_static.py
index 5f02305a5..85e177bea 100644
--- a/tests/test_web_typography_static.py
+++ b/tests/test_web_typography_static.py
@@ -452,3 +452,63 @@ def test_every_focus_visible_selector_gets_the_canonical_ring() -> None:
         "focus there is either invisible or a second colour vocabulary "
         "(docs/DESIGN.md 'Focus'):\n" + "\n".join(f"  {s}" for s in unringed)
     )
+
+
+# ---------------------------------------------------------------------------
+# Horizontal overflow: the red scrollbar on Settings -> Advanced
+# ---------------------------------------------------------------------------
+
+
+def test_select_control_clips_its_value() -> None:
+    """Pins the owner-visible defect "Settings -> Advanced paints a horizontal
+    scrollbar in the desktop app": WebKit computes `overflow: visible` on a
+    native select (Blink's UA sheet clips it), so a long selected option leaks
+    out of the control and into the page scroller."""
+    css = _decommented(_read("web/ui.css"))
+    bodies = [
+        body for selector, body in RULE.findall(css)
+        if selector.strip() == "select.ui-control"
+    ]
+    assert bodies, "no `select.ui-control` rule in web/ui.css"
+    values = [
+        part.split(":", 1)[1].strip().lower()
+        for body in bodies
+        for part in body.split(";")
+        if part.strip() and part.split(":", 1)[0].strip() == "overflow"
+    ]
+    assert values, (
+        "`select.ui-control` declares no `overflow`, so WebKit lets a long "
+        "selected option paint past the control's own box and widen the "
+        "settings scroller"
+    )
+    assert all(value != "visible" for value in values), (
+        f"`select.ui-control` re-opens the clip: overflow {values}"
+    )
+
+
+def test_webkit_scrollbar_recipe_covers_both_axes() -> None:
+    """Pins the owner-visible defect "the scrollbar is thick and red": the
+    global `::-webkit-scrollbar` recipe sized only `width`, which is the
+    VERTICAL bar, so any horizontal bar kept the 16-17px UA thickness while
+    still wearing the accent thumb."""
+    css = _decommented(_read("web/style.css"))
+    bodies = [
+        body for selector, body in RULE.findall(css)
+        if selector.strip() == "::-webkit-scrollbar"
+    ]
+    assert bodies, "no global `::-webkit-scrollbar` rule in web/style.css"
+    declarations: dict[str, str] = {}
+    for body in bodies:
+        for part in body.split(";"):
+            if ":" not in part:
+                continue
+            name, _, value = part.partition(":")
+            declarations[name.strip().lower()] = value.strip().lower()
+    assert "width" in declarations and "height" in declarations, (
+        "the global scrollbar recipe must size both axes; it declares "
+        f"{sorted(declarations)}"
+    )
+    assert declarations["width"] == declarations["height"], (
+        "the horizontal bar must be as thin as the vertical one: "
+        f"width {declarations['width']} vs height {declarations['height']}"
+    )
diff --git a/web/style.css b/web/style.css
index 71246349d..c2b20100a 100644
--- a/web/style.css
+++ b/web/style.css
@@ -3503,8 +3503,11 @@ textarea.chat-input::placeholder { color: var(--text-muted); }
     gap: 6px;
 }
 
-::-webkit-scrollbar { width: 6px; }
+/* `width` is the vertical bar, `height` the horizontal one: without the height
+   a horizontal bar falls back to the 16-17px UA default and paints thick. */
+::-webkit-scrollbar { width: 6px; height: 6px; }
 ::-webkit-scrollbar-track { background: transparent; }
+::-webkit-scrollbar-corner { background: transparent; }
 ::-webkit-scrollbar-thumb { background: var(--accent-18); border-radius: var(--radius-xs); }
 ::-webkit-scrollbar-thumb:hover { background: var(--accent-35); }
 
diff --git a/web/ui.css b/web/ui.css
index 46c5d6d81..4ec063fa7 100644
--- a/web/ui.css
+++ b/web/ui.css
@@ -377,9 +377,14 @@
     box-shadow: 0 0 0 3px var(--focus-accent-ring);
 }
 .ui-control:disabled { color: var(--text-disabled); cursor: not-allowed; }
+/* WebKit (the packaged desktop shell) computes `overflow: visible` on a native
+   select, so a long selected option paints past the control and turns the page
+   scroller into a horizontal one; the clip keeps it inside the control's box. */
 select.ui-control {
     -webkit-appearance: none;
     appearance: none;
+    overflow: hidden;
+    text-overflow: ellipsis;
     padding-inline-end: calc(var(--space-3) * 3);
     background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='14' height='14' viewBox='0 0 14 14'%3E%3Cpath d='M3 5l4 4 4-4' fill='none' stroke='%23e2e8f0' stroke-width='1.5' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E");
     background-repeat: no-repeat;