diff --git a/CLAUDE.md b/CLAUDE.md
index aa1141b..2f54f3b 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -36,8 +36,11 @@ A Manifest V3 Chrome extension (TypeScript + React 18 + Vite + Tailwind). Three
 - `src/lib/annotate.ts` — `exportAnnotatedImage` rasterizes annotations onto the screenshot; `selectFeedbackRenderMode` picks footer vs. overlay so the export canvas never exceeds `MAX_EXPORT_CANVAS_HEIGHT`/`AREA` limits.
 - `src/lib/feedback.ts` — `buildExternalLlmPrompt` (the structured prompt copied for the cloud-LLM fallback) and `annotationSummary`.
 - `src/lib/boxResize.ts` — box drag/resize geometry.
+- `src/lib/selectNavigation.ts` — `getNextIndex`/`matchTypeahead` keyboard-navigation rules for the custom `Select` listbox (so the interaction logic is testable apart from the DOM).
 - `src/types/annotation.ts` — the `Annotation` discriminated union (`box` | `arrow` | `text`).
 
+**Design system (`src/components/ui/*` + `src/styles/globals.css` + `tailwind.config.js`):** components are driven by semantic HSL **CSS-variable tokens** (`--primary`, `--secondary`, `--muted`, `--accent`, `--destructive` + `-hover`, `--border`, `--input`, `--ring`) mapped to Tailwind color utilities — use `bg-primary`/`border-input`/`ring-ring` etc., never hardcoded `emerald-*`/`slate-*` literals, in the primitives. A `.dark` token block exists (opt-in via `class="dark"`; light is the default) and is kept **outside `@layer base`** so Tailwind does not tree-shake the unreferenced selector. `Select` is a **custom WAI-ARIA listbox** (not a native `<select>`): pass `value` + `onValueChange` + `options`, not `<option>` children — the native option popup is unstylable, which is why it was replaced.
+
 **Two outputs from the editor:** (1) *Copy Local Share Link* → `saveLocalShare` + viewer URL; (2) *Prepare for Cloud LLM* → downloads the annotated PNG and copies `buildExternalLlmPrompt` output to the clipboard. The extension makes **no network requests of its own**; data leaves the device only via that explicit manual export.
 
 ## Conventions
diff --git a/src/components/ui/select.tsx b/src/components/ui/select.tsx
index 8b9aa2c..39e65a0 100644
--- a/src/components/ui/select.tsx
+++ b/src/components/ui/select.tsx
@@ -1,34 +1,248 @@
 import * as React from "react";
+import { useCallback, useEffect, useId, useRef, useState } from "react";
 import { cn } from "@/lib/utils";
+import { getNextIndex, matchTypeahead, type ListboxKey } from "@/lib/selectNavigation";
 
-export type SelectProps = React.SelectHTMLAttributes<HTMLSelectElement>;
+export interface SelectOption {
+  value: string;
+  label: string;
+}
+
+export interface SelectProps {
+  value: string;
+  onValueChange: (value: string) => void;
+  options: SelectOption[];
+  className?: string;
+  disabled?: boolean;
+  id?: string;
+  "aria-label"?: string;
+  "aria-labelledby"?: string;
+}
+
+const TYPEAHEAD_RESET_MS = 600;
+
+function Select({
+  value,
+  onValueChange,
+  options,
+  className,
+  disabled,
+  id,
+  "aria-label": ariaLabel,
+  "aria-labelledby": ariaLabelledby
+}: SelectProps): JSX.Element {
+  const [open, setOpen] = useState(false);
+  const [activeIndex, setActiveIndex] = useState(-1);
+
+  const rootRef = useRef<HTMLDivElement | null>(null);
+  const buttonRef = useRef<HTMLButtonElement | null>(null);
+  const listRef = useRef<HTMLUListElement | null>(null);
+  const typeahead = useRef<{ query: string; timer: number | null }>({ query: "", timer: null });
+
+  const baseId = useId();
+  const listboxId = `${baseId}-listbox`;
+  const optionId = (index: number): string => `${baseId}-option-${index}`;
+
+  const selectedIndex = options.findIndex((option) => option.value === value);
+  const selectedLabel = selectedIndex >= 0 ? options[selectedIndex].label : "";
+
+  const openList = useCallback((): void => {
+    setActiveIndex(selectedIndex >= 0 ? selectedIndex : 0);
+    setOpen(true);
+  }, [selectedIndex]);
+
+  const closeList = useCallback((focusButton = true): void => {
+    setOpen(false);
+    setActiveIndex(-1);
+    if (focusButton) buttonRef.current?.focus();
+  }, []);
+
+  const commit = useCallback(
+    (index: number): void => {
+      const option = options[index];
+      if (option) onValueChange(option.value);
+      closeList();
+    },
+    [options, onValueChange, closeList]
+  );
+
+  // Close when a pointer goes down outside the component.
+  useEffect(() => {
+    if (!open) return;
+    const onPointerDown = (event: PointerEvent): void => {
+      if (rootRef.current && !rootRef.current.contains(event.target as Node)) {
+        setOpen(false);
+        setActiveIndex(-1);
+      }
+    };
+    document.addEventListener("pointerdown", onPointerDown);
+    return () => document.removeEventListener("pointerdown", onPointerDown);
+  }, [open]);
+
+  // Keep the active option scrolled into view (by index — option ids contain
+  // `:` from useId and are not valid CSS selectors).
+  useEffect(() => {
+    if (!open || activeIndex < 0) return;
+    const el = listRef.current?.children.item(activeIndex) as HTMLElement | null;
+    el?.scrollIntoView({ block: "nearest" });
+  }, [open, activeIndex]);
+
+  // Clear any pending typeahead reset timer on unmount.
+  useEffect(() => {
+    const state = typeahead.current;
+    return () => {
+      if (state.timer) window.clearTimeout(state.timer);
+    };
+  }, []);
+
+  const runTypeahead = useCallback(
+    (char: string): void => {
+      const state = typeahead.current;
+      state.query += char;
+      if (state.timer) window.clearTimeout(state.timer);
+      state.timer = window.setTimeout(() => {
+        state.query = "";
+        state.timer = null;
+      }, TYPEAHEAD_RESET_MS);
+
+      const match = matchTypeahead(
+        options.map((option) => option.label),
+        state.query,
+        activeIndex >= 0 ? activeIndex : 0
+      );
+      if (match >= 0) setActiveIndex(match);
+    },
+    [options, activeIndex]
+  );
+
+  const onKeyDown = (event: React.KeyboardEvent<HTMLButtonElement>): void => {
+    if (disabled) return;
+    const { key } = event;
+
+    if (!open) {
+      if (key === "ArrowDown" || key === "ArrowUp" || key === "Enter" || key === " ") {
+        event.preventDefault();
+        openList();
+      }
+      return;
+    }
+
+    switch (key) {
+      case "ArrowDown":
+      case "ArrowUp":
+      case "Home":
+      case "End":
+        event.preventDefault();
+        setActiveIndex((current) => getNextIndex(current, options.length, key as ListboxKey));
+        return;
+      case "Enter":
+      case " ":
+        event.preventDefault();
+        if (activeIndex >= 0) commit(activeIndex);
+        return;
+      case "Escape":
+        event.preventDefault();
+        closeList();
+        return;
+      case "Tab":
+        // Let focus leave naturally; just dismiss the list.
+        setOpen(false);
+        setActiveIndex(-1);
+        return;
+      default:
+        if (key.length === 1 && !event.metaKey && !event.ctrlKey && !event.altKey) {
+          event.preventDefault();
+          runTypeahead(key);
+        }
+    }
+  };
 
-const Select = React.forwardRef<HTMLSelectElement, SelectProps>(({ className, ...props }, ref) => {
   return (
-    <div className="relative">
-      <select
+    <div ref={rootRef} className="relative">
+      <button
+        ref={buttonRef}
+        type="button"
+        id={id}
+        role="combobox"
+        aria-haspopup="listbox"
+        aria-expanded={open}
+        aria-controls={listboxId}
+        aria-activedescendant={open && activeIndex >= 0 ? optionId(activeIndex) : undefined}
+        aria-label={ariaLabel}
+        aria-labelledby={ariaLabelledby}
+        disabled={disabled}
+        onClick={() => (open ? closeList() : openList())}
+        onKeyDown={onKeyDown}
         className={cn(
-          "flex h-10 w-full appearance-none rounded-md border border-input bg-card px-3 py-2 pr-9 text-sm text-foreground shadow-sm transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50 aria-[invalid=true]:border-destructive aria-[invalid=true]:focus-visible:ring-destructive",
+          "flex h-10 w-full items-center justify-between gap-2 rounded-md border border-input bg-card px-3 py-2 text-left text-sm text-foreground shadow-sm transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50",
           className
         )}
-        ref={ref}
-        {...props}
-      />
-      <svg
-        aria-hidden="true"
-        viewBox="0 0 24 24"
-        fill="none"
-        stroke="currentColor"
-        strokeWidth="2"
-        strokeLinecap="round"
-        strokeLinejoin="round"
-        className="pointer-events-none absolute right-3 top-1/2 size-4 -translate-y-1/2 text-muted-foreground"
       >
-        <path d="m6 9 6 6 6-6" />
-      </svg>
+        <span className="truncate">{selectedLabel}</span>
+        <svg
+          aria-hidden="true"
+          viewBox="0 0 24 24"
+          fill="none"
+          stroke="currentColor"
+          strokeWidth="2"
+          strokeLinecap="round"
+          strokeLinejoin="round"
+          className={cn(
+            "size-4 shrink-0 text-muted-foreground transition-transform duration-200 ease-swift",
+            open && "rotate-180"
+          )}
+        >
+          <path d="m6 9 6 6 6-6" />
+        </svg>
+      </button>
+
+      {open ? (
+        <ul
+          ref={listRef}
+          id={listboxId}
+          role="listbox"
+          aria-label={ariaLabel}
+          aria-labelledby={ariaLabelledby}
+          className="absolute left-0 right-0 top-full z-20 mt-1 max-h-60 overflow-auto rounded-md border border-border bg-card p-1 shadow-lg"
+        >
+          {options.map((option, index) => {
+            const selected = option.value === value;
+            const active = index === activeIndex;
+            return (
+              <li
+                key={option.value}
+                id={optionId(index)}
+                role="option"
+                aria-selected={selected}
+                onPointerEnter={() => setActiveIndex(index)}
+                onClick={() => commit(index)}
+                className={cn(
+                  "flex cursor-pointer items-center justify-between gap-2 rounded-sm px-2.5 py-1.5 text-sm",
+                  active ? "bg-primary text-primary-foreground" : "text-foreground"
+                )}
+              >
+                <span className="truncate">{option.label}</span>
+                {selected ? (
+                  <svg
+                    aria-hidden="true"
+                    viewBox="0 0 24 24"
+                    fill="none"
+                    stroke="currentColor"
+                    strokeWidth="2.5"
+                    strokeLinecap="round"
+                    strokeLinejoin="round"
+                    className="size-4 shrink-0"
+                  >
+                    <path d="M20 6 9 17l-5-5" />
+                  </svg>
+                ) : null}
+              </li>
+            );
+          })}
+        </ul>
+      ) : null}
     </div>
   );
-});
-Select.displayName = "Select";
+}
 
 export { Select };
diff --git a/src/editor/main.tsx b/src/editor/main.tsx
index d80733f..1d0522b 100644
--- a/src/editor/main.tsx
+++ b/src/editor/main.tsx
@@ -587,32 +587,42 @@ function EditorApp(): JSX.Element {
           </Button>
         </CardHeader>
         <CardContent className="space-y-3">
-          <label className="block space-y-1.5">
-            <span className="text-xs font-semibold uppercase tracking-wide text-slate-500">
+          <div className="space-y-1.5">
+            <span
+              id="interaction-label"
+              className="block text-xs font-semibold uppercase tracking-wide text-slate-500"
+            >
               Interaction
             </span>
             <Select
+              aria-labelledby="interaction-label"
               value={interactionMode}
-              onChange={(event) => setInteractionMode(event.target.value as "draw" | "move")}
-            >
-              <option value="draw">Draw New</option>
-              <option value="move">Move Existing</option>
-            </Select>
-          </label>
+              onValueChange={(value) => setInteractionMode(value as "draw" | "move")}
+              options={[
+                { value: "draw", label: "Draw New" },
+                { value: "move", label: "Move Existing" }
+              ]}
+            />
+          </div>
 
-          <label className="block space-y-1.5">
-            <span className="text-xs font-semibold uppercase tracking-wide text-slate-500">
+          <div className="space-y-1.5">
+            <span
+              id="tool-label"
+              className="block text-xs font-semibold uppercase tracking-wide text-slate-500"
+            >
               Tool
             </span>
             <Select
+              aria-labelledby="tool-label"
               value={tool}
-              onChange={(event) => setTool(event.target.value as AnnotationTool)}
-            >
-              <option value="box">Box</option>
-              <option value="arrow">Arrow</option>
-              <option value="text">Text</option>
-            </Select>
-          </label>
+              onValueChange={(value) => setTool(value as AnnotationTool)}
+              options={[
+                { value: "box", label: "Box" },
+                { value: "arrow", label: "Arrow" },
+                { value: "text", label: "Text" }
+              ]}
+            />
+          </div>
 
           <label className="block space-y-1.5">
             <span className="text-xs font-semibold uppercase tracking-wide text-slate-500">
diff --git a/src/lib/selectNavigation.ts b/src/lib/selectNavigation.ts
new file mode 100644
index 0000000..cf9ce94
--- /dev/null
+++ b/src/lib/selectNavigation.ts
@@ -0,0 +1,43 @@
+/**
+ * Pure keyboard-navigation helpers for the custom listbox Select.
+ * Kept free of React/DOM so the interaction rules can be unit tested.
+ */
+
+export type ListboxKey = "ArrowDown" | "ArrowUp" | "Home" | "End";
+
+/**
+ * Compute the next active option index for a listbox key press. `current` may be
+ * -1 (nothing active yet). Movement is clamped to the list (no wrap-around), which
+ * matches native <select> behaviour.
+ */
+export function getNextIndex(current: number, count: number, key: ListboxKey): number {
+  if (count <= 0) return -1;
+
+  switch (key) {
+    case "ArrowDown":
+      return current < 0 ? 0 : Math.min(current + 1, count - 1);
+    case "ArrowUp":
+      return current < 0 ? count - 1 : Math.max(current - 1, 0);
+    case "Home":
+      return 0;
+    case "End":
+      return count - 1;
+  }
+}
+
+/**
+ * Typeahead match: find the next option whose label starts with `query`
+ * (case-insensitive), searching circularly from index `from` (inclusive).
+ * Returns -1 when nothing matches or the query is empty.
+ */
+export function matchTypeahead(labels: string[], query: string, from: number): number {
+  const q = query.trim().toLowerCase();
+  if (!q || labels.length === 0) return -1;
+
+  const start = from < 0 ? 0 : from % labels.length;
+  for (let i = 0; i < labels.length; i += 1) {
+    const index = (start + i) % labels.length;
+    if (labels[index].toLowerCase().startsWith(q)) return index;
+  }
+  return -1;
+}
diff --git a/tests/selectNavigation.test.ts b/tests/selectNavigation.test.ts
new file mode 100644
index 0000000..db856a9
--- /dev/null
+++ b/tests/selectNavigation.test.ts
@@ -0,0 +1,49 @@
+import { describe, expect, it } from "vitest";
+import { getNextIndex, matchTypeahead } from "../src/lib/selectNavigation";
+
+describe("getNextIndex", () => {
+  it("moves down and clamps at the last option", () => {
+    expect(getNextIndex(0, 3, "ArrowDown")).toBe(1);
+    expect(getNextIndex(2, 3, "ArrowDown")).toBe(2);
+  });
+
+  it("moves up and clamps at the first option", () => {
+    expect(getNextIndex(2, 3, "ArrowUp")).toBe(1);
+    expect(getNextIndex(0, 3, "ArrowUp")).toBe(0);
+  });
+
+  it("opens to the first item on ArrowDown and last on ArrowUp when nothing is active", () => {
+    expect(getNextIndex(-1, 3, "ArrowDown")).toBe(0);
+    expect(getNextIndex(-1, 3, "ArrowUp")).toBe(2);
+  });
+
+  it("jumps to the ends with Home and End", () => {
+    expect(getNextIndex(1, 3, "Home")).toBe(0);
+    expect(getNextIndex(1, 3, "End")).toBe(2);
+  });
+
+  it("returns -1 for an empty list", () => {
+    expect(getNextIndex(0, 0, "ArrowDown")).toBe(-1);
+  });
+});
+
+describe("matchTypeahead", () => {
+  const labels = ["Box", "Arrow", "Text"];
+
+  it("matches a prefix case-insensitively", () => {
+    expect(matchTypeahead(labels, "ar", 0)).toBe(1);
+    expect(matchTypeahead(labels, "T", 0)).toBe(2);
+  });
+
+  it("searches circularly from the given index", () => {
+    // Two options start with the same letter; search wraps past `from`.
+    expect(matchTypeahead(["Apple", "Banana", "Apricot"], "a", 1)).toBe(2);
+    expect(matchTypeahead(["Apple", "Banana", "Apricot"], "a", 0)).toBe(0);
+  });
+
+  it("returns -1 when nothing matches or query is empty", () => {
+    expect(matchTypeahead(labels, "z", 0)).toBe(-1);
+    expect(matchTypeahead(labels, "", 0)).toBe(-1);
+    expect(matchTypeahead([], "a", 0)).toBe(-1);
+  });
+});