Improve Threads beta around degraded mode (#8318)

* Hide MAB Threads prompt if user would have degraded mode

* Confirm user wants to enable Threads beta if in degraded mode

* fix

* Fix copy

src/settings/controllers/ThreadBetaController.tsx

@@ -0,0 +1,53 @@
+/*
+Copyright 2022 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 * as React from "react";
+import { Thread } from "matrix-js-sdk/src/models/thread";
+
+import SettingController from "./SettingController";
+import PlatformPeg from "../../PlatformPeg";
+import { SettingLevel } from "../SettingLevel";
+import Modal from "../../Modal";
+import QuestionDialog from "../../components/views/dialogs/QuestionDialog";
+import { _t } from "../../languageHandler";
+
+export default class ThreadBetaController extends SettingController {
+    public async beforeChange(level: SettingLevel, roomId: string, newValue: any): Promise<boolean> {
+        if (Thread.hasServerSideSupport || !newValue) return true; // Full support or user is disabling
+
+        const { finished } = Modal.createTrackedDialog<[boolean]>("Thread beta", "degraded mode", QuestionDialog, {
+            title: _t("Partial Support for Threads"),
+            description: <>
+                <p>{ _t("Your homeserver does not currently support threads, so this feature may be unreliable. " +
+                    "Some threaded messages may not be reliably available. <a>Learn more</a>.", {}, {
+                    a: sub => (
+                        <a href="https://element.io/help#threads" target="_blank" rel="noreferrer noopener">{ sub }</a>
+                    ),
+                }) }</p>
+                <p>{ _t("Do you want to enable threads anyway?") }</p>
+            </>,
+            button: _t("Yes, enable"),
+        });
+        const [enable] = await finished;
+        return enable;
+    }
+
+    public onChange(level: SettingLevel, roomId: string, newValue: any) {
+        // Requires a reload as we change an option flag on the `js-sdk`
+        // And the entire sync history needs to be parsed again
+        PlatformPeg.get().reload();
+    }
+}