Arrow-key navigation for the combobox

Arrow keys cycle through the listed matches and create rows with
aria-activedescendant tracking, and Enter activates the armed row.
This closes a keyboard-only gap in quick-add: with matches still
listed, Enter deliberately refuses to mint from a half-typed name,
which left the create row reachable only by mouse.

web/src/styles.css

@@ -2076,6 +2076,7 @@ footer span:first-child {
   color: inherit;
 }
 .combo-option:hover,
+.combo-option.active,
 .combo-option[aria-selected="true"] {
   background: #f0f7f4;
 }

web/src/ui.tsx

@@ -141,6 +141,10 @@ export function Combobox({
   const [createError, setCreateError] = useState("");
   const [open, setOpen] = useState(false);
   const [query, setQuery] = useState("");
+  // Index into the interactive rows (matches first, then create rows); -1
+  // means no row is armed and Enter falls back to exact/single-match logic.
+  const [active, setActive] = useState(-1);
+  const listID = useId();
   const filter = query.trim().toLowerCase();
   const matches = options.filter((o) => o.label.toLowerCase().includes(filter));
   const exact = filter
@@ -152,6 +156,8 @@ export function Combobox({
   const selected = options.find((o) => o.value === value);
   const creations =
     create && filter && !exact && !disabled ? create(query.trim()) : [];
+  const total = shown.length + creations.length;
+  const cursor = active < total ? active : -1;
   const pick = (v: string) => {
     onChange(v);
     setOpen(false);
@@ -176,42 +182,64 @@ export function Combobox({
         role="combobox"
         aria-expanded={open}
         aria-autocomplete="list"
+        aria-controls={open ? listID : undefined}
+        aria-activedescendant={
+          open && cursor >= 0 ? `${listID}-${cursor}` : undefined
+        }
         disabled={disabled}
         value={open ? query : (selected?.label ?? value)}
         placeholder={placeholder}
         onFocus={() => {
           setQuery("");
+          setActive(-1);
           setOpen(true);
         }}
         onChange={(e) => {
           setQuery(e.target.value);
           setCreateError("");
+          setActive(-1);
           setOpen(true);
         }}
         onBlur={() => setOpen(false)}
         onKeyDown={(e) => {
           if (e.key === "Escape") setOpen(false);
+          if ((e.key === "ArrowDown" || e.key === "ArrowUp") && open && total) {
+            e.preventDefault();
+            setActive(
+              e.key === "ArrowDown"
+                ? (cursor + 1) % total
+                : (cursor <= 0 ? total : cursor) - 1,
+            );
+          }
           if (e.key === "Enter" && open) {
             e.preventDefault();
+            if (cursor >= 0 && cursor < shown.length) pick(shown[cursor].value);
+            else if (cursor >= shown.length)
+              void runCreate(creations[cursor - shown.length]);
+            else {
               const hit = exact ?? (shown.length === 1 ? shown[0] : undefined);
               if (hit) pick(hit.value);
-            // Enter creates only when nothing matches at all: with matches
+              // Without an armed row, Enter creates only when nothing
-            // still listed, minting from a half-typed name is too easy.
+              // matches at all: minting from a half-typed name is too easy.
               else if (!shown.length && creations.length === 1)
                 void runCreate(creations[0]);
             }
+          }
         }}
       />
       {adornment && !open && (
         <span className="combo-adornment">{adornment}</span>
       )}
       {open && (
-        <ul className="combo-options" role="listbox">
+        <ul className="combo-options" role="listbox" id={listID}>
-          {shown.map((o) => (
+          {shown.map((o, i) => (
             <li key={o.value}>
               <button
                 type="button"
-                className="combo-option"
+                id={`${listID}-${i}`}
+                className={
+                  i === cursor ? "combo-option active" : "combo-option"
+                }
                 role="option"
                 aria-selected={o.value === value}
                 onMouseDown={(e) => e.preventDefault()}
@@ -222,11 +250,16 @@ export function Combobox({
               </button>
             </li>
           ))}
-          {creations.map((c) => (
+          {creations.map((c, i) => (
             <li key={c.key}>
               <button
                 type="button"
-                className="combo-option create"
+                id={`${listID}-${shown.length + i}`}
+                className={
+                  shown.length + i === cursor
+                    ? "combo-option create active"
+                    : "combo-option create"
+                }
                 role="option"
                 aria-selected={false}
                 disabled={creating}