ui: add components

packages/ui/src/DropdownMenu/DropdownMenu.tsx

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+import clsx from 'clsx';
+import React, { Fragment } from 'react';
+import { Menu, Transition } from '@headlessui/react';
+import { ChevronDownIcon } from '@heroicons/react/24/solid';
+
+import DropdownMenuItem from './DropdownMenuItem';
+
+export type DropdownMenuAlignment = 'end' | 'start';
+export type DropdownMenuSize = 'inherit' | 'regular';
+
+type Props = Readonly<{
+  align?: DropdownMenuAlignment;
+  children: React.ReactNode; // TODO: Change to strict children.
+  label: React.ReactNode;
+  size?: DropdownMenuSize;
+}>;
+
+DropdownMenu.Item = DropdownMenuItem;
+
+const alignmentClasses: Record<DropdownMenuAlignment, string> = {
+  end: 'origin-top-right right-0',
+  start: 'origin-top-left left-0',
+};
+
+export default function DropdownMenu({
+  align = 'start',
+  children,
+  label,
+  size = 'regular',
+}: Props) {
+  return (
+    <Menu as="div" className="relative inline-block">
+      <div className="flex">
+        <Menu.Button
+          className={clsx(
+            'group inline-flex justify-center font-medium text-slate-700 hover:text-slate-900',
+            size === 'regular' && 'text-sm',
+          )}>
+          <div>{label}</div>
+          <ChevronDownIcon
+            aria-hidden="true"
+            className="-mr-1 ml-1 h-5 w-5 flex-shrink-0 text-slate-400 group-hover:text-slate-500"
+          />
+        </Menu.Button>
+      </div>
+      <Transition
+        as={Fragment}
+        enter="transition ease-out duration-100"
+        enterFrom="transform opacity-0 scale-95"
+        enterTo="transform opacity-100 scale-100"
+        leave="transition ease-in duration-75"
+        leaveFrom="transform opacity-100 scale-100"
+        leaveTo="transform opacity-0 scale-95">
+        <Menu.Items
+          className={clsx(
+            alignmentClasses[align],
+            'ring-primary-500 absolute z-10 mt-2 w-48 rounded-md bg-white shadow-lg ring-1 ring-opacity-5 focus:outline-none',
+          )}>
+          <div className="py-1">{children}</div>
+        </Menu.Items>
+      </Transition>
+    </Menu>
+  );
+}

packages/ui/src/DropdownMenu/DropdownMenuItem.tsx

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+import clsx from 'clsx';
+import React from 'react';
+
+import { Menu } from '@headlessui/react';
+
+type Props = Readonly<{
+  href?: string;
+  isSelected?: boolean;
+  label: React.ReactNode;
+  onClick: () => void;
+}>;
+
+export default function DropdownMenuItem({
+  href,
+  isSelected = false,
+  label,
+  onClick,
+}: Props) {
+  return (
+    <Menu.Item>
+      {({ active }) => {
+        const props = {
+          children: label,
+          className: clsx(
+            isSelected ? 'font-medium text-slate-900' : 'text-slate-500',
+            active && 'bg-slate-100',
+            'block px-4 py-2 text-sm w-full text-left',
+          ),
+          onClick,
+        };
+
+        if (href == null) {
+          return <button type="button" {...props} />;
+        }
+
+        // TODO: Change to <Link> when there's a need for client-side navigation.
+        return <a href={href} {...props} />;
+      }}
+    </Menu.Item>
+  );
+}