diff --git a/docs/swarm/ui.md b/docs/swarm/ui.md
index 26d77b00..054b04b0 100644
--- a/docs/swarm/ui.md
+++ b/docs/swarm/ui.md
@@ -11,6 +11,15 @@ The signed-in user's avatar sits at the right end of the header on every page.
 It opens a menu with the user's name and links to authelia's settings and
 sign-out pages; the hive dashboard and the per-agent page have the same menu.
 
+Every dropdown (a form's select, the refresh interval, the history window) opens a
+list in the swarm UI's own theme, not the browser's, and works the same in every
+current browser. Open it with a click, or with the arrow keys, **Enter** or
+**Space** while it has focus. Then the arrow keys, **Home**, **End**, **Page Up**
+and **Page Down** move through the options, and **Enter**, **Space** or a click take
+the highlighted one. **Escape**, **Tab** or a click elsewhere shut the list without
+changing anything. Typing the start of an option's name picks it, also with the list
+shut. A dropdown inside a dialog opens above the dialog and the dialog doesn't cut it off.
+
 | route                                          | what                                                                                                                                                                                                                                                                  |
 | ---------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `/`                                            | the start page: the swarm's name over a digital-rain backdrop, plus a paperclip offering to set up an agent while no `ruth` exists (it opens the create form with `ruth` filled in); the swarm name in the header links here                                          |
diff --git a/frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx b/frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx
index dce9f873..fbb0ceec 100644
--- a/frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx
+++ b/frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx
@@ -14,7 +14,7 @@
 //
 // `hive` field added because agent creation had no way to record which
 // hive an agent runs on. `POST /api/agents` now requires it, so the
-// roster fetched off `GET /api/hives` backs a required select here
+// roster fetched off `GET /api/hives` backs a required dropdown here
 // rather than a free-text field — a spawn target has to be one of the
 // swarm's actual hives, same reasoning `Ident.parse` gets client-side
 // pattern validation for `name`.
@@ -159,8 +159,8 @@ export function CreateAgentForm({
       setHives(data);
       // Single-hive swarms are the common case — default it rather than
       // making the operator pick the only option. Multi-hive swarms get
-      // no default (the placeholder stays selected), so `required` on
-      // the select below forces an explicit choice per mara's scope.
+      // no default (the placeholder stays selected), so the identity step
+      // cannot be left until one is picked (`stepProblem`), per mara's scope.
       if (data.length === 1) setHive(data[0].name);
     })().catch((e: unknown) => setHivesError({ detail: String(e) }));
   }, []);
diff --git a/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.css b/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.css
index 0e1a134d..bc7cde1e 100644
--- a/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.css
+++ b/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.css
@@ -13,7 +13,6 @@
   border-radius: 0.4em;
   color: var(--muted);
   font-size: 0.9em;
-  cursor: pointer;
 }
 .ui-refresh-picker:hover,
 .ui-refresh-picker:focus-within {
@@ -21,22 +20,21 @@
   background: var(--bg);
   color: var(--fg);
 }
-/* The select drives the interaction (native listbox, keyboard, a11y) but
-   contributes no chrome of its own — its own box is invisible, sized to
-   just its selected option's text, and its default arrow is stripped
-   since the chevron span stands in for it. */
-.ui-refresh-picker-select {
-  appearance: none;
+/* The Listbox contributes only its value and chevron: its own box is
+   invisible and as wide as the value, so the picker's border is the one
+   that shows. */
+.ui-refresh-picker .ui-listbox {
+  width: auto;
+}
+.ui-refresh-picker-select.ui-form-control {
+  width: auto;
+  min-height: 2.5em;
+  padding: 0;
   border: none;
   background: none;
   color: inherit;
-  font: inherit;
-  padding: 0;
-  margin: 0;
-  cursor: pointer;
 }
-.ui-refresh-picker-icon,
-.ui-refresh-picker-chevron {
+.ui-refresh-picker-icon {
   font-size: 0.85em;
   line-height: 1;
 }
diff --git a/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.tsx b/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.tsx
index a42cb1b8..9dc97977 100644
--- a/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.tsx
+++ b/frontend/packages/swarm-ui/src/ui/refresh-interval/RefreshInterval.tsx
@@ -14,18 +14,17 @@
 // passive-until-touched setting that lives in a panel's title row
 // (`Panel`'s `actions` slot), not a form. Visually quiet (no border/
 // fill until hovered/focused) the same way `LinksMenu`'s header button
-// is, so it reads as chrome rather than another input to fill in. A
-// native `<select>` still drives it (no custom dropdown/listbox to
-// reinvent) — `appearance: none` strips its default arrow so the
-// chevron span can stand in for it, and the visible "value" text is
-// just whichever `<option>` the browser is already rendering as
-// selected, not a second copy of the label kept in sync by hand.
+// is, so it reads as chrome rather than another input to fill in. The
+// dropdown is the shared `Listbox` (its list is themed, not the browser's),
+// restyled to have no chrome of its own: the chevron is the Listbox's, and the
+// visible value is its selected option.
 //
 // The hook, not the picker, owns re-fetch safety: a caller passes its
 // own fetch callback, and it's the CALLER's job to make sure that
 // callback doesn't clobber an input the operator is mid-edit on — this
 // hook only decides *when* to call it.
 import { useEffect, useRef } from "preact/hooks";
+import { Listbox } from "../select-field/Listbox.js";
 import "./RefreshInterval.css";
 
 // `null` means "off" throughout this module — no separate boolean, so
@@ -51,37 +50,23 @@ export function RefreshIntervalPicker({
 }) {
   const current = PRESETS.find((p) => p.value === value) ?? PRESETS[0];
   return (
-    // A `<label>` wrapper (not a bare `<span>`) so the icon/chevron
-    // padding is part of the click/tap target too, not just the native
-    // select box itself — same touch-target-floor reasoning as the rest
-    // of the kit, just without a visible label string to hang it off
-    // of (hence the `aria-label` on the `<select>` for the accessible
-    // name a sighted-only icon+chevron can't supply).
-    <label class="ui-refresh-picker" for={id}>
+    <div class="ui-refresh-picker">
       <span class="ui-refresh-picker-icon" aria-hidden="true">
         🕐
       </span>
-      <select
+      <Listbox
         id={id}
         class="ui-refresh-picker-select"
-        aria-label="refresh interval"
+        ariaLabel="refresh interval"
         value={current.label}
-        onChange={(e) => {
-          const label = (e.target as HTMLSelectElement).value;
+        options={PRESETS.map((p) => ({ value: p.label, label: p.label }))}
+        minListWidth={96}
+        onChange={(label) => {
           const preset = PRESETS.find((p) => p.label === label);
           if (preset) onChange(preset.value);
         }}
-      >
-        {PRESETS.map((p) => (
-          <option key={p.label} value={p.label}>
-            {p.label}
-          </option>
-        ))}
-      </select>
-      <span class="ui-refresh-picker-chevron" aria-hidden="true">
-        ⌄
-      </span>
-    </label>
+      />
+    </div>
   );
 }
 
diff --git a/frontend/packages/swarm-ui/src/ui/select-field/Listbox.css b/frontend/packages/swarm-ui/src/ui/select-field/Listbox.css
new file mode 100644
index 00000000..c386952f
--- /dev/null
+++ b/frontend/packages/swarm-ui/src/ui/select-field/Listbox.css
@@ -0,0 +1,84 @@
+/* <Listbox>: the control is `.ui-form-control`'s box (FormField.css) laid out as
+   value + chevron, and the list is a themed box in the top layer. */
+.ui-listbox {
+  position: relative;
+  width: 100%;
+}
+.ui-select-trigger {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.5em;
+  text-align: start;
+  cursor: pointer;
+}
+.ui-select-trigger:disabled {
+  cursor: default;
+  opacity: 0.6;
+}
+.ui-select-trigger[aria-expanded="true"] {
+  border-color: var(--purple);
+}
+.ui-select-value {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.ui-select-empty {
+  color: var(--muted);
+}
+/* A chevron drawn with borders, so it takes the text colour and needs no glyph
+   a font might lack. */
+.ui-select-chevron {
+  flex: none;
+  width: 0.45em;
+  height: 0.45em;
+  border-right: 2px solid currentColor;
+  border-bottom: 2px solid currentColor;
+  transform: translateY(-0.12em) rotate(45deg);
+  opacity: 0.7;
+}
+.ui-select-trigger[aria-expanded="true"] .ui-select-chevron {
+  transform: translateY(0.1em) rotate(225deg);
+}
+
+/* A popover's own UA styles centre it with `inset: 0; margin: auto`; this puts
+   it where `placePopup` says. */
+.ui-select-list {
+  position: fixed;
+  inset: auto;
+  margin: 0;
+  padding: 0.25em;
+  box-sizing: border-box;
+  overflow-y: auto;
+  scrollbar-width: thin;
+  scrollbar-color: var(--purple-dim) transparent;
+  list-style: none;
+  background: var(--bg-elev);
+  color: var(--fg);
+  border: 1px solid var(--purple-dim);
+  border-radius: 0.4em;
+  box-shadow: 0 0.25em 0.75em rgba(0, 0, 0, 0.3);
+  font: inherit;
+}
+.ui-select-option {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.75em;
+  min-height: 2.5em;
+  padding: 0.3em 0.6em;
+  border-radius: 0.3em;
+  cursor: pointer;
+}
+.ui-select-option-active {
+  background: var(--bg);
+}
+.ui-select-option-selected {
+  font-weight: 600;
+}
+.ui-select-option-selected::after {
+  content: "✓";
+  color: var(--purple);
+}
diff --git a/frontend/packages/swarm-ui/src/ui/select-field/Listbox.tsx b/frontend/packages/swarm-ui/src/ui/select-field/Listbox.tsx
new file mode 100644
index 00000000..df5d3401
--- /dev/null
+++ b/frontend/packages/swarm-ui/src/ui/select-field/Listbox.tsx
@@ -0,0 +1,269 @@
+// <Listbox> — the dropdown a native `<select>` cannot be: its open list is
+// drawn by the page, so it wears the theme. `SelectField` and the refresh
+// picker are built on it. Same job as a select (pick one of a few options), so
+// it follows the WAI-ARIA "select-only combobox" pattern: the control is a
+// button with `role="combobox"` that keeps focus, and the list is a
+// `role="listbox"` whose current option the control names through
+// `aria-activedescendant`.
+//
+// Keyboard. Closed: Arrow Up/Down, Enter and Space open it; typing picks the
+// option starting with those letters. Open: Arrow Up/Down, Home, End, Page Up
+// and Page Down move, Enter and Space take the highlighted option, Escape and
+// Tab close it without changing anything.
+//
+// The list is a `popover="manual"` element, so it sits in the browser's top
+// layer: it is not clipped by a scrolling ancestor, and inside a modal
+// `<dialog>` it shows above the dialog (it is a descendant of the control, so
+// the dialog's inertness leaves it alone). `position: fixed` coordinates come
+// from the control's box (`placePopup`). A scroll or resize elsewhere, or a
+// click outside, closes it, as a native select's list closes.
+//
+// Not a native control, so there is no form validation: `required` is only
+// `aria-required`, and a caller that needs a choice checks it itself.
+import { useEffect, useLayoutEffect, useRef, useState } from "preact/hooks";
+import {
+  indexOfValue,
+  moveIndex,
+  nextQuery,
+  placePopup,
+  typeAhead,
+  type ListboxOption,
+  type Placement,
+} from "./select-nav.js";
+import "./Listbox.css";
+
+export function Listbox({
+  id,
+  value,
+  options,
+  onChange,
+  disabled,
+  required,
+  placeholder,
+  ariaLabel,
+  labelledBy,
+  class: extraClass,
+  minListWidth,
+}: {
+  id: string;
+  value: string;
+  options: ListboxOption[];
+  onChange: (value: string) => void;
+  disabled?: boolean;
+  required?: boolean;
+  /** Shown on the control while `value` matches no option. */
+  placeholder?: string;
+  /** The accessible name when there is no visible label. */
+  ariaLabel?: string;
+  /** The id of the element that is the visible label. */
+  labelledBy?: string;
+  /** Extra class on the control, for a compact look. */
+  class?: string;
+  /** The list is at least this wide in px, when the control is narrower. */
+  minListWidth?: number;
+}) {
+  const [open, setOpen] = useState(false);
+  const [active, setActive] = useState(-1);
+  const [place, setPlace] = useState<Placement | null>(null);
+  const triggerRef = useRef<HTMLButtonElement>(null);
+  const listRef = useRef<HTMLUListElement>(null);
+  const typed = useRef({ query: "", at: 0 });
+  const listId = `${id}-list`;
+  const optionId = (i: number) => `${id}-opt-${i}`;
+  const selected = indexOfValue(options, value);
+  const shown = selected >= 0 ? options[selected].label : (placeholder ?? "");
+
+  function openList() {
+    if (disabled || options.length === 0) return;
+    setActive(selected >= 0 ? selected : 0);
+    setOpen(true);
+  }
+
+  function closeList() {
+    setOpen(false);
+    setPlace(null);
+  }
+
+  function take(i: number) {
+    if (i >= 0 && i < options.length && options[i].value !== value) {
+      onChange(options[i].value);
+    }
+    closeList();
+    triggerRef.current?.focus();
+  }
+
+  // Show the list in the top layer, measure it, and place it.
+  useLayoutEffect(() => {
+    const list = listRef.current;
+    const trigger = triggerRef.current;
+    if (!list || !trigger) return;
+    if (!open) {
+      if (list.matches(":popover-open")) list.hidePopover();
+      return;
+    }
+    if (!list.matches(":popover-open")) list.showPopover();
+    const r = trigger.getBoundingClientRect();
+    setPlace(
+      placePopup(
+        { top: r.top, bottom: r.bottom, left: r.left, width: r.width },
+        { width: window.innerWidth, height: window.innerHeight },
+        // The content, plus the border the box-sizing includes.
+        list.scrollHeight + list.offsetHeight - list.clientHeight,
+        minListWidth,
+      ),
+    );
+  }, [open, minListWidth]);
+
+  // Keep the highlighted option in view as the keys move it.
+  useEffect(() => {
+    if (open && place) {
+      listRef.current?.children[active]?.scrollIntoView({ block: "nearest" });
+    }
+  }, [open, place, active]);
+
+  // A click elsewhere, a scroll of anything but the list, or a resize closes it.
+  useEffect(() => {
+    if (!open) return;
+    function onPointerDown(e: PointerEvent) {
+      const t = e.target as Node;
+      if (!triggerRef.current?.contains(t) && !listRef.current?.contains(t))
+        closeList();
+    }
+    function onScroll(e: Event) {
+      if (!listRef.current?.contains(e.target as Node)) closeList();
+    }
+    document.addEventListener("pointerdown", onPointerDown);
+    window.addEventListener("scroll", onScroll, true);
+    window.addEventListener("resize", closeList);
+    return () => {
+      document.removeEventListener("pointerdown", onPointerDown);
+      window.removeEventListener("scroll", onScroll, true);
+      window.removeEventListener("resize", closeList);
+    };
+  }, [open]);
+
+  function onKeyDown(e: KeyboardEvent) {
+    if (disabled || e.altKey || e.ctrlKey || e.metaKey) return;
+    const key = e.key;
+    const typing = key.length === 1 && key !== " ";
+    if (typing) {
+      const now = Date.now();
+      const query = nextQuery(typed.current.query, key, now - typed.current.at);
+      typed.current = { query, at: now };
+      const hit = typeAhead(options, open ? active : selected, query);
+      if (hit >= 0) {
+        e.preventDefault();
+        if (open) setActive(hit);
+        else if (options[hit].value !== value) onChange(options[hit].value);
+      }
+      return;
+    }
+    if (!open) {
+      if (["ArrowDown", "ArrowUp", "Enter", " ", "Home", "End"].includes(key)) {
+        e.preventDefault();
+        openList();
+        if (key === "Home") setActive(0);
+        if (key === "End") setActive(options.length - 1);
+      }
+      return;
+    }
+    switch (key) {
+      case "ArrowDown":
+      case "ArrowUp":
+      case "Home":
+      case "End":
+      case "PageDown":
+      case "PageUp":
+        e.preventDefault();
+        setActive(moveIndex(active, options.length, key));
+        break;
+      case "Enter":
+      case " ":
+        e.preventDefault();
+        take(active);
+        break;
+      case "Escape":
+        e.preventDefault();
+        // Only the list: a dialog around it must not close as well.
+        e.stopPropagation();
+        closeList();
+        break;
+      case "Tab":
+        closeList();
+        break;
+    }
+  }
+
+  return (
+    <div class="ui-listbox">
+      <button
+        type="button"
+        id={id}
+        ref={triggerRef}
+        class={
+          "ui-form-control ui-select-trigger" +
+          (extraClass ? " " + extraClass : "")
+        }
+        role="combobox"
+        aria-haspopup="listbox"
+        aria-expanded={open}
+        aria-controls={listId}
+        aria-activedescendant={
+          open && active >= 0 ? optionId(active) : undefined
+        }
+        aria-required={required ? true : undefined}
+        aria-label={ariaLabel}
+        aria-labelledby={labelledBy}
+        disabled={disabled}
+        onClick={() => (open ? closeList() : openList())}
+        onKeyDown={onKeyDown}
+      >
+        <span
+          class={"ui-select-value" + (selected < 0 ? " ui-select-empty" : "")}
+        >
+          {shown}
+        </span>
+        <span class="ui-select-chevron" aria-hidden="true" />
+      </button>
+      <ul
+        id={listId}
+        ref={listRef}
+        class="ui-select-list"
+        role="listbox"
+        aria-labelledby={labelledBy}
+        aria-label={ariaLabel}
+        {...{ popover: "manual" }}
+        style={
+          place
+            ? {
+                top: `${place.top}px`,
+                left: `${place.left}px`,
+                width: `${place.width}px`,
+                maxHeight: `${place.maxHeight}px`,
+              }
+            : { visibility: "hidden" }
+        }
+      >
+        {options.map((o, i) => (
+          <li
+            key={o.value}
+            id={optionId(i)}
+            role="option"
+            aria-selected={i === selected}
+            class={
+              "ui-select-option" +
+              (i === active ? " ui-select-option-active" : "") +
+              (i === selected ? " ui-select-option-selected" : "")
+            }
+            // A press must not take focus off the control.
+            onPointerDown={(e) => e.preventDefault()}
+            onPointerMove={() => i !== active && setActive(i)}
+            onClick={() => take(i)}
+          >
+            {o.label}
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}
diff --git a/frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx b/frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx
index cd2b0a3c..6d120bda 100644
--- a/frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx
+++ b/frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx
@@ -1,10 +1,12 @@
-// <SelectField> — a labelled `<select>`, `TextField`'s sibling for the
+// <SelectField> — a labelled dropdown (`Listbox`, which draws its own themed
+// list instead of the browser's), `TextField`'s sibling for the
 // "pick one of these" shape (the hive-choice dropdown the create-agent
 // form needs is the motivating caller). Options are plain
 // value/label pairs, not `ComponentChildren` — every real caller so far
 // has flat string options, and generic children would need a second
 // primitive (`SelectField.Option`) for zero real benefit today.
 import { FormField } from "../form-field/FormField.js";
+import { Listbox } from "./Listbox.js";
 
 export interface SelectOption {
   value: string;
@@ -26,36 +28,31 @@ export function SelectField({
   value: string;
   onChange: (value: string) => void;
   options: SelectOption[];
+  // Only `aria-required`: the control is not a native select, so it has no
+  // form validation. A caller that needs a choice checks it itself.
   required?: boolean;
   disabled?: boolean;
-  // Rendered as a disabled, always-first `value=""` option — for a
-  // required select with no default (e.g. a multi-hive roster: no
-  // single right answer to pre-select), so the control shows something
-  // other than silently landing on whatever option happens to be first.
-  // Omit when the field always has a real default (`value` never `''`).
+  // Shown on the control while `value` matches no option (it is not a
+  // listed option) — for a required select with no default (e.g. a
+  // multi-hive roster: no single right answer to pre-select), so the
+  // control shows something other than silently landing on whatever
+  // option happens to be first. Omit when the field always has a real
+  // default (`value` never `''`).
   placeholder?: string;
 }) {
+  const labelId = `${id}-label`;
   return (
-    <FormField label={label} htmlFor={id}>
-      <select
+    <FormField label={label} labelId={labelId}>
+      <Listbox
         id={id}
-        class="ui-form-control"
+        labelledBy={labelId}
         value={value}
+        options={options}
+        onChange={onChange}
         required={required}
         disabled={disabled}
-        onChange={(e) => onChange((e.target as HTMLSelectElement).value)}
-      >
-        {placeholder !== undefined && (
-          <option value="" disabled>
-            {placeholder}
-          </option>
-        )}
-        {options.map((o) => (
-          <option key={o.value} value={o.value}>
-            {o.label}
-          </option>
-        ))}
-      </select>
+        placeholder={placeholder}
+      />
     </FormField>
   );
 }
diff --git a/frontend/packages/swarm-ui/src/ui/select-field/select-nav.ts b/frontend/packages/swarm-ui/src/ui/select-field/select-nav.ts
new file mode 100644
index 00000000..6e6e7607
--- /dev/null
+++ b/frontend/packages/swarm-ui/src/ui/select-field/select-nav.ts
@@ -0,0 +1,132 @@
+// What <Listbox> decides without a DOM: which option the arrow keys, Home and
+// End land on, which one typing picks, and where the list opens. Plain
+// functions of their arguments, so `npm test` can run them.
+
+export interface ListboxOption {
+  value: string;
+  label: string;
+}
+
+/** The index of the option holding `value`, or -1. */
+export function indexOfValue(options: ListboxOption[], value: string): number {
+  return options.findIndex((o) => o.value === value);
+}
+
+// How far Page Up and Page Down jump.
+const PAGE = 10;
+
+/**
+ * The active option after `key`, among `count` options. Arrow keys stop at the
+ * ends, they do not wrap. A key that moves nothing leaves `active` as it was.
+ */
+export function moveIndex(active: number, count: number, key: string): number {
+  if (count <= 0) return -1;
+  const last = count - 1;
+  switch (key) {
+    case "ArrowDown":
+      return Math.min(active + 1, last);
+    case "ArrowUp":
+      return Math.max(active - 1, 0);
+    case "PageDown":
+      return Math.min(active + PAGE, last);
+    case "PageUp":
+      return Math.max(active - PAGE, 0);
+    case "Home":
+      return 0;
+    case "End":
+      return last;
+    default:
+      return active;
+  }
+}
+
+/** How long a pause ends a typed word, in milliseconds. */
+export const TYPE_AHEAD_MS = 700;
+
+/**
+ * The text typed so far once `key` is pressed: it continues the last word when
+ * the pause since the previous key was short, and starts a new one otherwise.
+ */
+export function nextQuery(
+  previous: string,
+  key: string,
+  gapMs: number,
+): string {
+  return gapMs > TYPE_AHEAD_MS ? key : previous + key;
+}
+
+/**
+ * The option typing `query` picks, searching on from `from` and wrapping
+ * round, or -1 when none starts with it. One letter, or that letter typed over
+ * and over, steps through the options that start with it; a longer word stays
+ * on the current option while it still matches.
+ */
+export function typeAhead(
+  options: ListboxOption[],
+  from: number,
+  query: string,
+): number {
+  const q = query.toLowerCase();
+  if (q === "" || options.length === 0) return -1;
+  const repeated = q.length > 1 && [...q].every((c) => c === q[0]);
+  const needle = repeated ? q[0] : q;
+  const start = needle.length === 1 ? from + 1 : Math.max(from, 0);
+  for (let step = 0; step < options.length; step++) {
+    const i = (start + step) % options.length;
+    if (options[i].label.toLowerCase().startsWith(needle)) return i;
+  }
+  return -1;
+}
+
+export interface Box {
+  top: number;
+  bottom: number;
+  left: number;
+  width: number;
+}
+
+export interface Placement {
+  top: number;
+  left: number;
+  width: number;
+  maxHeight: number;
+}
+
+// Space kept between the list and the viewport's edge, and between the list
+// and the control.
+const EDGE_PX = 8;
+const GAP_PX = 4;
+
+/**
+ * Where a list `contentHeight` tall opens for a control at `trigger`: under it
+ * when it fits, else above it if that has more room, else under it with a
+ * scroll. At least as wide as the control, and kept inside the viewport.
+ */
+export function placePopup(
+  trigger: Box,
+  viewport: { width: number; height: number },
+  contentHeight: number,
+  minWidth = 0,
+): Placement {
+  const below = Math.max(
+    viewport.height - trigger.bottom - EDGE_PX - GAP_PX,
+    0,
+  );
+  const above = Math.max(trigger.top - EDGE_PX - GAP_PX, 0);
+  const openUp = contentHeight > below && above > below;
+  const maxHeight = Math.min(contentHeight, openUp ? above : below);
+  const width = Math.min(
+    Math.max(trigger.width, minWidth),
+    Math.max(viewport.width - 2 * EDGE_PX, 0),
+  );
+  const left = Math.min(
+    Math.max(trigger.left, EDGE_PX),
+    Math.max(viewport.width - EDGE_PX - width, EDGE_PX),
+  );
+  return {
+    top: openUp ? trigger.top - GAP_PX - maxHeight : trigger.bottom + GAP_PX,
+    left,
+    width,
+    maxHeight,
+  };
+}
diff --git a/frontend/packages/swarm-ui/test/select-nav.test.mjs b/frontend/packages/swarm-ui/test/select-nav.test.mjs
new file mode 100644
index 00000000..7ae05b1d
--- /dev/null
+++ b/frontend/packages/swarm-ui/test/select-nav.test.mjs
@@ -0,0 +1,113 @@
+// What the styled dropdown's keys and placement do. Run by `npm test`.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  indexOfValue,
+  moveIndex,
+  nextQuery,
+  placePopup,
+  TYPE_AHEAD_MS,
+  typeAhead,
+} from "../src/ui/select-field/select-nav.ts";
+
+const opts = ["alpha", "beta", "bravo", "charlie", "Delta"].map((label) => ({
+  value: label.toLowerCase(),
+  label,
+}));
+
+test("the option holding a value is found, or -1", () => {
+  assert.equal(indexOfValue(opts, "bravo"), 2);
+  assert.equal(indexOfValue(opts, "nope"), -1);
+});
+
+test("arrow keys step and stop at the ends; Home and End jump", () => {
+  assert.equal(moveIndex(1, 5, "ArrowDown"), 2);
+  assert.equal(moveIndex(4, 5, "ArrowDown"), 4, "stops at the last");
+  assert.equal(moveIndex(1, 5, "ArrowUp"), 0);
+  assert.equal(moveIndex(0, 5, "ArrowUp"), 0, "stops at the first");
+  assert.equal(moveIndex(2, 5, "Home"), 0);
+  assert.equal(moveIndex(2, 5, "End"), 4);
+  assert.equal(moveIndex(0, 30, "PageDown"), 10);
+  assert.equal(moveIndex(25, 30, "PageDown"), 29);
+  assert.equal(moveIndex(3, 30, "PageUp"), 0);
+});
+
+test("a key that moves nothing, or no options, leave the active one alone", () => {
+  assert.equal(moveIndex(2, 5, "a"), 2);
+  assert.equal(moveIndex(0, 0, "ArrowDown"), -1);
+  assert.equal(
+    moveIndex(-1, 3, "ArrowDown"),
+    0,
+    "from nothing active, the first",
+  );
+});
+
+test("typing a letter steps through the options starting with it, wrapping", () => {
+  assert.equal(typeAhead(opts, 0, "b"), 1);
+  assert.equal(typeAhead(opts, 1, "b"), 2);
+  assert.equal(typeAhead(opts, 2, "b"), 1, "wraps round to the first b");
+  assert.equal(typeAhead(opts, 0, "d"), 4, "case does not matter");
+  assert.equal(typeAhead(opts, 0, "z"), -1);
+});
+
+test("the same letter typed again steps on; a longer word narrows in place", () => {
+  assert.equal(typeAhead(opts, 1, "bb"), 2, "bb is b again");
+  assert.equal(typeAhead(opts, 1, "br"), 2);
+  assert.equal(typeAhead(opts, 2, "br"), 2, "stays while it still matches");
+  assert.equal(typeAhead(opts, 0, ""), -1);
+  assert.equal(typeAhead([], 0, "a"), -1);
+});
+
+test("a short pause continues the word, a long one starts a new one", () => {
+  assert.equal(nextQuery("b", "r", 100), "br");
+  assert.equal(nextQuery("br", "c", TYPE_AHEAD_MS + 1), "c");
+});
+
+const viewport = { width: 1000, height: 800 };
+
+test("the list opens under the control when it fits", () => {
+  const p = placePopup(
+    { top: 100, bottom: 140, left: 50, width: 200 },
+    viewport,
+    120,
+  );
+  assert.deepEqual(p, { top: 144, left: 50, width: 200, maxHeight: 120 });
+});
+
+test("it opens above when under is too short and above has more room", () => {
+  const p = placePopup(
+    { top: 700, bottom: 740, left: 50, width: 200 },
+    viewport,
+    300,
+  );
+  assert.equal(p.maxHeight, 300);
+  assert.equal(p.top, 700 - 4 - 300);
+});
+
+test("it scrolls under the control when neither side fits it", () => {
+  const p = placePopup(
+    { top: 100, bottom: 140, left: 50, width: 200 },
+    { width: 1000, height: 300 },
+    900,
+  );
+  // 300 - 140 - 8 - 4 under; 100 - 12 above: under has more here.
+  assert.equal(p.top, 144);
+  assert.equal(p.maxHeight, 148);
+});
+
+test("it is at least the minimum width and stays inside the viewport", () => {
+  const p = placePopup(
+    { top: 100, bottom: 140, left: 960, width: 30 },
+    viewport,
+    100,
+    120,
+  );
+  assert.equal(p.width, 120);
+  assert.equal(p.left, 1000 - 8 - 120);
+  const q = placePopup(
+    { top: 0, bottom: 40, left: -20, width: 100 },
+    viewport,
+    50,
+  );
+  assert.equal(q.left, 8);
+});