Pick sensible default option for phone country dropdown

src/components/views/auth/CountryDropdown.tsx

@@ -57,16 +57,30 @@ export default class CountryDropdown extends React.Component<IProps, IState> {
     public constructor(props: IProps) {
         super(props);
 
-        let defaultCountry: PhoneNumberCountryDefinition = COUNTRIES[0];
+        let defaultCountry: PhoneNumberCountryDefinition | undefined;
         const defaultCountryCode = SdkConfig.get("default_country_code");
         if (defaultCountryCode) {
             const country = COUNTRIES.find((c) => c.iso2 === defaultCountryCode.toUpperCase());
             if (country) defaultCountry = country;
         }
 
+        if (!defaultCountry) {
+            try {
+                const locale = new Intl.Locale(navigator.language ?? navigator.languages[0]);
+                const code = locale.region ?? locale.language ?? locale.baseName;
+                const displayNames = new Intl.DisplayNames(["en"], { type: "region" });
+                const displayName = displayNames.of(code).toUpperCase();
+                defaultCountry = COUNTRIES.find(
+                    (c) => c.iso2 === code.toUpperCase() || c.name.toUpperCase() === displayName,
+                );
+            } catch (e) {
+                console.warn("Failed to detect default locale", e);
+            }
+        }
+
         this.state = {
             searchQuery: "",
-            defaultCountry,
+            defaultCountry: defaultCountry ?? COUNTRIES[0],
         };
     }
 

test/components/views/auth/CountryDropdown-test.tsx

@@ -0,0 +1,45 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+import { render } from "@testing-library/react";
+
+import CountryDropdown from "../../../../src/components/views/auth/CountryDropdown";
+
+describe("CountryDropdown", () => {
+    describe("defaultCountry", () => {
+        it.each([
+            ["en-GB", 44],
+            ["en-ie", 353],
+            ["es-ES", 34],
+            ["fr", 33],
+            ["pl", 48],
+            ["de-DE", 49],
+        ])("should pick appropriate default country for %s", (language, defaultCountryCode) => {
+            const fn = jest.fn();
+
+            Object.defineProperty(navigator, "language", {
+                configurable: true,
+                get() {
+                    return language;
+                },
+            });
+            render(<CountryDropdown onOptionChange={fn} isSmall={false} showPrefix={false} />);
+
+            expect(fn).toHaveBeenCalledWith(expect.objectContaining({ prefix: defaultCountryCode.toString() }));
+        });
+    });
+});