Convert dropdown_menu state to Typescript (#25585)

app/javascript/mastodon/reducers/dropdown_menu.ts

@@ -0,0 +1,33 @@
+import { createReducer } from '@reduxjs/toolkit';
+
+import { closeDropdownMenu, openDropdownMenu } from '../actions/dropdown_menu';
+
+interface DropdownMenuState {
+  openId: string | null;
+  keyboard: boolean;
+  scrollKey: string | null;
+}
+
+const initialState: DropdownMenuState = {
+  openId: null,
+  keyboard: false,
+  scrollKey: null,
+};
+
+export const dropdownMenuReducer = createReducer(initialState, (builder) => {
+  builder
+    .addCase(
+      openDropdownMenu,
+      (state, { payload: { id, keyboard, scrollKey } }) => {
+        state.openId = id;
+        state.keyboard = keyboard;
+        state.scrollKey = scrollKey;
+      },
+    )
+    .addCase(closeDropdownMenu, (state, { payload: { id } }) => {
+      if (state.openId === id) {
+        state.openId = null;
+        state.scrollKey = null;
+      }
+    });
+});