diff --git a/gui/src/pages/ClaudeCode.tsx b/gui/src/pages/ClaudeCode.tsx
index ef02006348b..e479965a7c2 100644
--- a/gui/src/pages/ClaudeCode.tsx
+++ b/gui/src/pages/ClaudeCode.tsx
@@ -230,6 +230,7 @@ export default function ClaudeCode({ apiBase }: { apiBase: string }) {
             onChange={v => setState({ ...state, authMode: v as ClaudeCodeState["authMode"] })}
             label={t("claude.authMode")}
             style={{ minWidth: 220 }}
+            portal
           />
         </div>
 
@@ -282,6 +283,7 @@ export default function ClaudeCode({ apiBase }: { apiBase: string }) {
               onChange={v => setState({ ...state, autoCompactWindow: v === "" ? null : Number(v) })}
               label={t("claude.autoCompactWindow")}
               style={{ minWidth: 130 }}
+              portal
             />
           </div>
         )}
@@ -320,6 +322,7 @@ export default function ClaudeCode({ apiBase }: { apiBase: string }) {
                       : { ...override, backend: value === "auto" ? undefined : value as SidecarBackend },
                   })}
                   label={t("dash.sidecarBackend")}
+                  portal
                 />
                 <input
                   className="input mono"
diff --git a/gui/src/select-position.ts b/gui/src/select-position.ts
new file mode 100644
index 00000000000..e5294512acd
--- /dev/null
+++ b/gui/src/select-position.ts
@@ -0,0 +1,98 @@
+import type { CSSProperties } from "react";
+
+export interface SelectMenuTriggerRect {
+  top: number;
+  bottom: number;
+  left: number;
+  right: number;
+  width: number;
+  height: number;
+}
+
+export interface SelectMenuStyleOptions {
+  align?: "left" | "right";
+  placement?: "below" | "right";
+  menuHeight?: number;
+}
+
+const MENU_GAP_PX = 4;
+const FLIP_GAP_PX = 8;
+const VIEWPORT_PAD_PX = 8;
+const MAX_MENU_HEIGHT_PX = 280;
+const MIN_MENU_HEIGHT_PX = 120;
+const BESIDE_MIN_WIDTH_PX = 160;
+
+function viewportHeight() {
+  return typeof window !== "undefined" ? window.innerHeight : 800;
+}
+
+function viewportWidth() {
+  return typeof window !== "undefined" ? window.innerWidth : 1024;
+}
+
+export function computeSelectMenuStyle(
+  trigger: SelectMenuTriggerRect,
+  { align = "left", placement = "below", menuHeight = MAX_MENU_HEIGHT_PX }: SelectMenuStyleOptions = {},
+): CSSProperties {
+  const measuredHeight = Math.min(Math.max(menuHeight, MIN_MENU_HEIGHT_PX), MAX_MENU_HEIGHT_PX);
+  const vh = viewportHeight();
+  const vw = viewportWidth();
+
+  if (placement === "right") {
+    const spaceBelow = vh - trigger.top - VIEWPORT_PAD_PX;
+    const spaceAbove = trigger.top - VIEWPORT_PAD_PX;
+    const openAbove = measuredHeight + FLIP_GAP_PX > spaceBelow && spaceAbove > spaceBelow;
+    const left = Math.max(VIEWPORT_PAD_PX, Math.min(trigger.right + 6, vw - BESIDE_MIN_WIDTH_PX - VIEWPORT_PAD_PX));
+
+    if (openAbove) {
+      return {
+        position: "fixed",
+        left,
+        bottom: vh - trigger.top + FLIP_GAP_PX,
+        minWidth: BESIDE_MIN_WIDTH_PX,
+        maxHeight: Math.max(MIN_MENU_HEIGHT_PX, Math.min(MAX_MENU_HEIGHT_PX, trigger.top - VIEWPORT_PAD_PX - MENU_GAP_PX)),
+      };
+    }
+
+    return {
+      position: "fixed",
+      top: trigger.top,
+      left,
+      minWidth: BESIDE_MIN_WIDTH_PX,
+      maxHeight: Math.max(MIN_MENU_HEIGHT_PX, Math.min(MAX_MENU_HEIGHT_PX, vh - trigger.top - VIEWPORT_PAD_PX)),
+    };
+  }
+
+  const width = Math.max(trigger.width, 0);
+  const spaceBelow = vh - trigger.bottom - VIEWPORT_PAD_PX;
+  const spaceAbove = trigger.top - VIEWPORT_PAD_PX;
+  const flipUp = measuredHeight + MENU_GAP_PX > spaceBelow && spaceAbove > spaceBelow;
+
+  if (flipUp) {
+    const style: CSSProperties = {
+      position: "fixed",
+      bottom: vh - trigger.top + FLIP_GAP_PX,
+      minWidth: width,
+      maxHeight: Math.max(MIN_MENU_HEIGHT_PX, spaceAbove - MENU_GAP_PX),
+    };
+    if (align === "right") {
+      style.right = vw - trigger.right;
+    } else {
+      style.left = Math.max(VIEWPORT_PAD_PX, Math.min(trigger.left, vw - VIEWPORT_PAD_PX - width));
+    }
+    return style;
+  }
+
+  const style: CSSProperties = {
+    position: "fixed",
+    top: trigger.bottom + MENU_GAP_PX,
+    minWidth: width,
+    maxHeight: Math.max(MIN_MENU_HEIGHT_PX, Math.min(MAX_MENU_HEIGHT_PX, spaceBelow - MENU_GAP_PX)),
+  };
+  if (align === "right") {
+    style.right = vw - trigger.right;
+  } else {
+    style.left = Math.max(VIEWPORT_PAD_PX, Math.min(trigger.left, vw - VIEWPORT_PAD_PX - width));
+  }
+  return style;
+}
diff --git a/gui/src/styles.css b/gui/src/styles.css
index a1fc2a85821..b4ce5408898 100644
--- a/gui/src/styles.css
+++ b/gui/src/styles.css
@@ -575,6 +575,8 @@ select.input { appearance: none; }
   border: 1px solid var(--border); border-radius: var(--radius);
   padding: 4px; box-shadow: 0 8px 32px rgb(0 0 0 / 0.16);
 }
+.select-dropdown-portal { position: fixed; }
+.select-dropdown-portal { z-index: 60; }
 .select-dropdown-right { left: auto !important; right: 0; }
 .select-dropdown-beside { top: 0; left: calc(100% + 6px); right: auto; min-width: 10rem; }
 .select-option {
diff --git a/gui/src/ui.tsx b/gui/src/ui.tsx
index b076477358e..c4b78dd1fc9 100644
--- a/gui/src/ui.tsx
+++ b/gui/src/ui.tsx
@@ -1,7 +1,9 @@
 /* Shared UI primitives built on the design-system classes in styles.css. */
-import { useEffect, useRef, useState, type CSSProperties, type ReactNode } from "react";
+import { useCallback, useEffect, useLayoutEffect, useRef, useState, type CSSProperties, type ReactNode } from "react";
+import { createPortal } from "react-dom";
 import { IconCheck, IconAlert } from "./icons";
 import { IconChevron } from "./icons";
+import { computeSelectMenuStyle } from "./select-position";
 
 export function Switch({ on, onClick, disabled, label }: { on: boolean; onClick: () => void; disabled?: boolean; label?: string }) {
   return (
@@ -23,7 +25,7 @@ export function Notice({ tone, children }: { tone: "ok" | "err"; children: React
 
 export interface SelectOption { value: string; label: React.ReactNode }
 
-export function Select({ value, options, onChange, disabled, label, style, align, placement, dropdownStyle }: {
+export function Select({ value, options, onChange, disabled, label, style, align, placement, dropdownStyle, portal = false }: {
   value: string;
   options: SelectOption[];
   onChange: (value: string) => void;
@@ -33,23 +35,91 @@ export function Select({ value, options, onChange, disabled, label, style, align
   align?: "left" | "right";
   placement?: "below" | "right";
   dropdownStyle?: CSSProperties;
+  portal?: boolean;
 }) {
   const [open, setOpen] = useState(false);
+  const [menuStyle, setMenuStyle] = useState<CSSProperties | undefined>();
   const ref = useRef<HTMLDivElement>(null);
+  const triggerRef = useRef<HTMLButtonElement>(null);
+  const menuRef = useRef<HTMLDivElement>(null);
   const current = options.find(o => o.value === value);
 
+  const reposition = useCallback((menuHeight?: number) => {
+    if (!portal) return;
+    const trigger = triggerRef.current;
+    if (!trigger) return;
+    setMenuStyle(computeSelectMenuStyle(trigger.getBoundingClientRect(), {
+      align,
+      placement,
+      menuHeight,
+    }));
+  }, [align, placement, portal]);
+
   useEffect(() => {
     if (!open) return;
-    const close = (e: MouseEvent) => { if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false); };
+    const close = (e: MouseEvent) => {
+      const target = e.target as Node;
+      if (ref.current?.contains(target) || menuRef.current?.contains(target)) return;
+      setOpen(false);
+    };
     const esc = (e: KeyboardEvent) => { if (e.key === "Escape") setOpen(false); };
     document.addEventListener("mousedown", close);
     document.addEventListener("keydown", esc);
     return () => { document.removeEventListener("mousedown", close); document.removeEventListener("keydown", esc); };
   }, [open]);
 
+  useLayoutEffect(() => {
+    if (!open || !portal) return;
+    reposition();
+    const onViewportChange = () => reposition(menuRef.current?.offsetHeight);
+    window.addEventListener("resize", onViewportChange);
+    window.addEventListener("scroll", onViewportChange, true);
+    return () => {
+      window.removeEventListener("resize", onViewportChange);
+      window.removeEventListener("scroll", onViewportChange, true);
+    };
+  }, [open, options.length, portal, reposition]);
+
+  useLayoutEffect(() => {
+    if (!open || !portal || !menuRef.current || !triggerRef.current) return;
+    const nextHeight = menuRef.current.offsetHeight;
+    if (!nextHeight) return;
+    const nextStyle = computeSelectMenuStyle(triggerRef.current.getBoundingClientRect(), {
+      align,
+      placement,
+      menuHeight: nextHeight,
+    });
+    setMenuStyle(prev => {
+      if (prev?.top === nextStyle.top && prev?.bottom === nextStyle.bottom && prev?.maxHeight === nextStyle.maxHeight) return prev;
+      return nextStyle;
+    });
+  }, [align, open, options.length, placement, portal]);
+
+  const dropdown = open ? (
+    <div
+      ref={menuRef}
+      className={`select-dropdown${portal ? " select-dropdown-portal" : ""}${align === "right" ? " select-dropdown-right" : ""}${placement === "right" ? " select-dropdown-beside" : ""}`}
+      role="listbox"
+      aria-label={label}
+      style={portal ? { ...menuStyle, zIndex: 60, ...dropdownStyle } : dropdownStyle}
+    >
+      {options.map(o => (
+        <button
+          key={o.value}
+          type="button"
+          role="option"
+          aria-selected={o.value === value}
+          className={`select-option${o.value === value ? " active" : ""}`}
+          onClick={() => { onChange(o.value); setOpen(false); }}
+        >{o.label}</button>
+      ))}
+    </div>
+  ) : null;
+
   return (
     <div ref={ref} className="custom-select" style={{ position: "relative", display: "inline-block", ...style }}>
       <button
+        ref={triggerRef}
         type="button"
         className="select-trigger"
         onClick={() => !disabled && setOpen(o => !o)}
@@ -61,20 +131,7 @@ export function Select({ value, options, onChange, disabled, label, style, align
         <span>{current?.label ?? value}</span>
         <IconChevron style={{ width: 12, height: 12, color: "var(--muted)", transform: open ? "rotate(90deg)" : "none", transition: "transform .12s" }} />
       </button>
-      {open && (
-        <div className={`select-dropdown${align === "right" ? " select-dropdown-right" : ""}${placement === "right" ? " select-dropdown-beside" : ""}`} role="listbox" aria-label={label} style={dropdownStyle}>
-          {options.map(o => (
-            <button
-              key={o.value}
-              type="button"
-              role="option"
-              aria-selected={o.value === value}
-              className={`select-option${o.value === value ? " active" : ""}`}
-              onClick={() => { onChange(o.value); setOpen(false); }}
-            >{o.label}</button>
-          ))}
-        </div>
-      )}
+      {portal ? (dropdown && createPortal(dropdown, document.body)) : dropdown}
     </div>
   );
 }
diff --git a/gui/tests/select-portal.test.tsx b/gui/tests/select-portal.test.tsx
new file mode 100644
index 00000000000..2bf9db98442
--- /dev/null
+++ b/gui/tests/select-portal.test.tsx
@@ -0,0 +1,83 @@
+import { afterEach, beforeEach, expect, test } from "bun:test";
+import { Window } from "happy-dom";
+import { act } from "react";
+import type { Root } from "react-dom/client";
+import { Select } from "../src/ui";
+
+// Mounted regressions for #340 / PR #393: the portal fix must (1) render an opt-in portaled
+// dropdown under document.body (outside the clipping card) with fixed positioning, and (2) leave a
+// NON-portal Select (the language menu contract) as a descendant of .custom-select keeping the
+// .select-dropdown-beside class so its glass fallback + mobile upward-placement CSS still apply.
+
+const globals = ["document", "window", "navigator", "IS_REACT_ACT_ENVIRONMENT"] as const;
+let previousGlobals: Record<(typeof globals)[number], unknown>;
+let testWindow: Window;
+
+beforeEach(() => {
+  previousGlobals = Object.fromEntries(globals.map((key) => [key, Reflect.get(globalThis, key)])) as typeof previousGlobals;
+  testWindow = new Window({ url: "http://localhost/" });
+  Object.defineProperties(globalThis, {
+    document: { configurable: true, value: testWindow.document },
+    window: { configurable: true, value: testWindow },
+    navigator: { configurable: true, value: testWindow.navigator },
+  });
+  (globalThis as typeof globalThis & { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+});
+
+afterEach(() => {
+  testWindow.close();
+  for (const key of globals) {
+    Object.defineProperty(globalThis, key, { configurable: true, value: previousGlobals[key] });
+  }
+});
+
+const OPTIONS = [
+  { value: "a", label: "Alpha" },
+  { value: "b", label: "Beta" },
+];
+
+async function mountAndOpen(node: React.ReactElement): Promise<{ container: HTMLElement; root: Root }> {
+  const { createRoot } = await import("react-dom/client");
+  // A bounded, clipping card the Claude settings dropdowns live inside.
+  const card = document.createElement("div");
+  card.className = "settings-card";
+  card.style.overflow = "hidden";
+  document.body.append(card);
+  let root!: Root;
+  await act(async () => {
+    root = createRoot(card);
+    root.render(node);
+  });
+  // Open the dropdown by clicking the trigger.
+  const trigger = card.querySelector<HTMLButtonElement>("button.select-trigger");
+  await act(async () => { trigger?.dispatchEvent(new testWindow.MouseEvent("click", { bubbles: true }) as unknown as MouseEvent); });
+  return { container: card, root };
+}
+
+test("a portal Select renders its dropdown under document.body, outside the clipping card", async () => {
+  const { container, root } = await mountAndOpen(
+    <Select value="a" options={OPTIONS} onChange={() => {}} label="Backend" portal />,
+  );
+  const listbox = document.body.querySelector<HTMLElement>('[role="listbox"]');
+  expect(listbox).not.toBeNull();
+  // The dropdown must NOT be nested inside the clipping card (that's the whole point of #340).
+  expect(container.contains(listbox)).toBe(false);
+  // Portaled dropdowns get the portal marker class and fixed positioning.
+  expect(listbox?.className).toContain("select-dropdown-portal");
+  await act(async () => { root.unmount(); });
+});
+
+test("a non-portal Select (language-menu contract) stays inside .custom-select and keeps .select-dropdown-beside", async () => {
+  const { container, root } = await mountAndOpen(
+    <Select value="a" options={OPTIONS} onChange={() => {}} label="Language" placement="right" />,
+  );
+  // Not portaled: nothing lands directly under body outside the mount container.
+  const bodyListboxes = Array.from(document.body.querySelectorAll<HTMLElement>('[role="listbox"]'));
+  expect(bodyListboxes.length).toBe(1);
+  const listbox = bodyListboxes[0];
+  // It remains a descendant of the .custom-select wrapper (contextual glass + mobile rules apply).
+  expect(container.querySelector(".custom-select")?.contains(listbox)).toBe(true);
+  expect(listbox.className).toContain("select-dropdown-beside");
+  expect(listbox.className).not.toContain("select-dropdown-portal");
+  await act(async () => { root.unmount(); });
+});
diff --git a/gui/tests/select-position.test.ts b/gui/tests/select-position.test.ts
new file mode 100644
index 00000000000..fd3d7e9b770
--- /dev/null
+++ b/gui/tests/select-position.test.ts
@@ -0,0 +1,117 @@
+import { afterEach, beforeEach, expect, test } from "bun:test";
+import { Window } from "happy-dom";
+import { computeSelectMenuStyle } from "../src/select-position";
+
+const globals = ["window"] as const;
+let previousGlobals: Record<(typeof globals)[number], unknown>;
+let testWindow: Window;
+
+beforeEach(() => {
+  previousGlobals = Object.fromEntries(globals.map((key) => [key, Reflect.get(globalThis, key)])) as typeof previousGlobals;
+  testWindow = new Window({ url: "http://localhost/", width: 1024, height: 800 });
+  Object.defineProperty(globalThis, "window", { configurable: true, value: testWindow });
+});
+
+afterEach(() => {
+  testWindow.close();
+  for (const key of globals) {
+    Object.defineProperty(globalThis, key, { configurable: true, value: previousGlobals[key] });
+  }
+});
+
+test("opens below by default when there is room under the trigger", () => {
+  const style = computeSelectMenuStyle({
+    top: 120,
+    bottom: 156,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { menuHeight: 180 });
+  expect(style.top).toBe(160);
+  expect(style.bottom).toBeUndefined();
+  expect(style.left).toBe(120);
+  expect(style.minWidth).toBe(200);
+});
+
+test("flips upward near the bottom of the viewport", () => {
+  const style = computeSelectMenuStyle({
+    top: 700,
+    bottom: 736,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { menuHeight: 180 });
+  expect(style.bottom).toBe(108);
+  expect(style.top).toBeUndefined();
+  expect(style.maxHeight).toBe(688);
+});
+
+test("right placement opens beside the trigger", () => {
+  const style = computeSelectMenuStyle({
+    top: 120,
+    bottom: 156,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { placement: "right", menuHeight: 120 });
+  expect(style.left).toBe(326);
+  expect(style.top).toBe(120);
+  expect(style.minWidth).toBe(160);
+});
+
+test("right placement flips above when the trigger is near the bottom edge", () => {
+  const style = computeSelectMenuStyle({
+    top: 700,
+    bottom: 736,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { placement: "right", menuHeight: 220 });
+  expect(style.left).toBe(326);
+  expect(style.bottom).toBe(108);
+  expect(style.top).toBeUndefined();
+});
+
+test("right placement clamps horizontally when the menu would overflow the viewport", () => {
+  const style = computeSelectMenuStyle({
+    top: 120,
+    bottom: 156,
+    left: 900,
+    right: 1000,
+    width: 100,
+    height: 36,
+  }, { placement: "right", menuHeight: 120 });
+  expect(style.left).toBe(856);
+  expect(style.top).toBe(120);
+});
+
+test("right placement flips above when the menu would overflow vertically below the trigger", () => {
+  const style = computeSelectMenuStyle({
+    top: 700,
+    bottom: 736,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { placement: "right", menuHeight: 280 });
+  expect(style.left).toBe(326);
+  expect(style.bottom).toBe(108);
+  expect(style.top).toBeUndefined();
+});
+
+test("right alignment anchors the menu to the trigger's right edge", () => {
+  const style = computeSelectMenuStyle({
+    top: 120,
+    bottom: 156,
+    left: 120,
+    right: 320,
+    width: 200,
+    height: 36,
+  }, { align: "right", menuHeight: 120 });
+  expect(style.right).toBe(704);
+  expect(style.left).toBeUndefined();
+});