Disable chat effects when reduce motion preferred

Some users prefer reduced motion, so this disables chat effects when such a
preference is set in the browser or OS.

src/settings/Settings.ts

@@ -37,6 +37,7 @@ import UIFeatureController from "./controllers/UIFeatureController";
 import { UIFeature } from "./UIFeature";
 import { OrderedMultiController } from "./controllers/OrderedMultiController";
 import {Layout} from "./Layout";
+import ReducedMotionController from './controllers/ReducedMotionController';
 
 // These are just a bunch of helper arrays to avoid copy/pasting a bunch of times
 const LEVELS_ROOM_SETTINGS = [
@@ -652,6 +653,7 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         displayName: _td("Show chat effects (animations when receiving e.g. confetti)"),
         default: true,
+        controller: new ReducedMotionController(),
     },
     "Widgets.pinned": { // deprecated
         supportedLevels: LEVELS_ROOM_OR_ACCOUNT,

src/settings/controllers/ReducedMotionController.ts

@@ -0,0 +1,45 @@
+/*
+Copyright 2021 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 SettingController from "./SettingController";
+import { SettingLevel } from "../SettingLevel";
+
+/**
+ * For animation-like settings, this controller checks whether the user has
+ * indicated they prefer reduced motion via browser or OS level settings.
+ * If they have, this forces the setting value to false.
+ */
+export default class ReducedMotionController extends SettingController {
+    public getValueOverride(
+        level: SettingLevel,
+        roomId: string,
+        calculatedValue: any,
+        calculatedAtLevel: SettingLevel,
+    ): any {
+        if (this.prefersReducedMotion()) {
+            return false;
+        }
+        return null; // no override
+    }
+
+    public get settingDisabled(): boolean {
+        return this.prefersReducedMotion();
+    }
+
+    private prefersReducedMotion(): boolean {
+        return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
+    }
+}