Respect the system high contrast setting when using system theme

res/themes/light-high-contrast/css/_light-high-contrast.scss

@@ -2,6 +2,7 @@
 $accent: #268075;
 $alert: #D62C25;
 $links: #0A6ECA;
+$primary-content: #17191C;
 $secondary-content: #5E6266;
 $tertiary-content: $secondary-content;
 $quaternary-content: $secondary-content;
@@ -106,3 +107,11 @@ $roomtopic-color: $secondary-content;
 .mx_FontScalingPanel_fontSlider {
     background-color: $roomlist-button-bg-color !important;
 }
+
+.mx_ThemeChoicePanel > .mx_ThemeSelectors > .mx_RadioButton input[type="radio"]:disabled + div {
+    border-color: $primary-content;
+}
+
+.mx_ThemeChoicePanel > .mx_ThemeSelectors > .mx_RadioButton.mx_RadioButton_disabled {
+    color: $primary-content;
+}

src/components/structures/UserMenu.tsx

@@ -32,7 +32,7 @@ import FeedbackDialog from "../views/dialogs/FeedbackDialog";
 import Modal from "../../Modal";
 import LogoutDialog from "../views/dialogs/LogoutDialog";
 import SettingsStore from "../../settings/SettingsStore";
-import { getCustomTheme } from "../../theme";
+import { findHighContrastTheme, getCustomTheme, isHighContrastTheme } from "../../theme";
 import AccessibleButton, { ButtonEvent } from "../views/elements/AccessibleButton";
 import SdkConfig from "../../SdkConfig";
 import { getHomePageUrl } from "../../utils/pages";
@@ -69,6 +69,7 @@ type PartialDOMRect = Pick<DOMRect, "width" | "left" | "top" | "height">;
 interface IState {
     contextMenuPosition: PartialDOMRect;
     isDarkTheme: boolean;
+    isHighContrast: boolean;
     selectedSpace?: Room;
     pendingRoomJoin: Set<string>;
 }
@@ -87,6 +88,7 @@ export default class UserMenu extends React.Component<IProps, IState> {
         this.state = {
             contextMenuPosition: null,
             isDarkTheme: this.isUserOnDarkTheme(),
+            isHighContrast: this.isUserOnHighContrastTheme(),
             pendingRoomJoin: new Set<string>(),
         };
 
@@ -142,6 +144,18 @@ export default class UserMenu extends React.Component<IProps, IState> {
         }
     }
 
+    private isUserOnHighContrastTheme(): boolean {
+        if (SettingsStore.getValue("use_system_theme")) {
+            return window.matchMedia("(prefers-contrast: more)").matches;
+        } else {
+            const theme = SettingsStore.getValue("theme");
+            if (theme.startsWith("custom-")) {
+                return false;
+            }
+            return isHighContrastTheme(theme);
+        }
+    }
+
     private onProfileUpdate = async () => {
         // the store triggered an update, so force a layout update. We don't
         // have any state to store here for that to magically happen.
@@ -153,7 +167,11 @@ export default class UserMenu extends React.Component<IProps, IState> {
     };
 
     private onThemeChanged = () => {
-        this.setState({ isDarkTheme: this.isUserOnDarkTheme() });
+        this.setState(
+            {
+                isDarkTheme: this.isUserOnDarkTheme(),
+                isHighContrast: this.isUserOnHighContrastTheme(),
+            });
     };
 
     private onAction = (ev: ActionPayload) => {
@@ -221,7 +239,13 @@ export default class UserMenu extends React.Component<IProps, IState> {
         // Disable system theme matching if the user hits this button
         SettingsStore.setValue("use_system_theme", null, SettingLevel.DEVICE, false);
 
-        const newTheme = this.state.isDarkTheme ? "light" : "dark";
+        let newTheme = this.state.isDarkTheme ? "light" : "dark";
+        if (this.state.isHighContrast) {
+            const hcTheme = findHighContrastTheme(newTheme);
+            if (hcTheme) {
+                newTheme = hcTheme;
+            }
+        }
         SettingsStore.setValue("theme", null, SettingLevel.DEVICE, newTheme); // set at same level as Appearance tab
     };
 

src/settings/watchers/ThemeWatcher.ts

@@ -19,7 +19,7 @@ import SettingsStore from '../SettingsStore';
 import dis from '../../dispatcher/dispatcher';
 import { Action } from '../../dispatcher/actions';
 import ThemeController from "../controllers/ThemeController";
-import { setTheme } from "../../theme";
+import { findHighContrastTheme, setTheme } from "../../theme";
 import { ActionPayload } from '../../dispatcher/payloads';
 import { SettingLevel } from "../SettingLevel";
 
@@ -32,6 +32,7 @@ export default class ThemeWatcher {
 
     private preferDark: MediaQueryList;
     private preferLight: MediaQueryList;
+    private preferHighContrast: MediaQueryList;
 
     private currentTheme: string;
 
@@ -44,6 +45,7 @@ export default class ThemeWatcher {
         // we can get the tristate of dark/light/unsupported
         this.preferDark = (<any>global).matchMedia("(prefers-color-scheme: dark)");
         this.preferLight = (<any>global).matchMedia("(prefers-color-scheme: light)");
+        this.preferHighContrast = (<any>global).matchMedia("(prefers-contrast: more)");
 
         this.currentTheme = this.getEffectiveTheme();
     }
@@ -54,6 +56,7 @@ export default class ThemeWatcher {
         if (this.preferDark.addEventListener) {
             this.preferDark.addEventListener('change', this.onChange);
             this.preferLight.addEventListener('change', this.onChange);
+            this.preferHighContrast.addEventListener('change', this.onChange);
         }
         this.dispatcherRef = dis.register(this.onAction);
     }
@@ -62,6 +65,7 @@ export default class ThemeWatcher {
         if (this.preferDark.addEventListener) {
             this.preferDark.removeEventListener('change', this.onChange);
             this.preferLight.removeEventListener('change', this.onChange);
+            this.preferHighContrast.removeEventListener('change', this.onChange);
         }
         SettingsStore.unwatchSetting(this.systemThemeWatchRef);
         SettingsStore.unwatchSetting(this.themeWatchRef);
@@ -108,8 +112,10 @@ export default class ThemeWatcher {
             SettingLevel.DEVICE, "use_system_theme", null, false, true);
         if (systemThemeExplicit) {
             logger.log("returning explicit system theme");
-            if (this.preferDark.matches) return 'dark';
-            if (this.preferLight.matches) return 'light';
+            const theme = this.themeBasedOnSystem();
+            if (theme) {
+                return theme;
+            }
         }
 
         // If the user has specifically enabled the theme (without the system matching option being
@@ -125,13 +131,31 @@ export default class ThemeWatcher {
         // If the user hasn't really made a preference in either direction, assume the defaults of the
         // settings and use those.
         if (SettingsStore.getValue('use_system_theme')) {
-            if (this.preferDark.matches) return 'dark';
-            if (this.preferLight.matches) return 'light';
+            const theme = this.themeBasedOnSystem();
+            if (theme) {
+                return theme;
+            }
         }
         logger.log("returning theme value");
         return SettingsStore.getValue('theme');
     }
 
+    private themeBasedOnSystem() {
+        let newTheme: string;
+        if (this.preferDark.matches) {
+            newTheme = 'dark';
+        } else if (this.preferLight.matches) {
+            newTheme = 'light';
+        }
+        if (this.preferHighContrast.matches) {
+            const hcTheme = findHighContrastTheme(newTheme);
+            if (hcTheme) {
+                newTheme = hcTheme;
+            }
+        }
+        return newTheme;
+    }
+
     public isSystemThemeSupported() {
         return this.preferDark.matches || this.preferLight.matches;
     }