Implement UI for history visibility acknowledgement. (#31156)

* feat: Implement UI for history visibility acknowledgement.

Shows a banner above the message composer whenever a user opens a room
with non-join history visibility, which they can dismiss.

- Whenever a user opens an encrypted room with non-join history
  visibility, show them a banner, unless we have already marked it as
  dismissed.
- Whenever a user opens an encrypted room with joined history
  visibility, we unmark it as dismissed.

Issue: https://github.com/element-hq/element-meta/issues/2875

* tests: Add test suite for `RoomStatusBarHistoryVisible`.

* docs: Document `RoomStatusBarHistoryVisible` and props interface.

* feat: Use newer `@vector-im/compound` components.

* test: Update snapshots for `RoomStatusBarHistoryVisible` tests.

* chore: Update playwright screenshots.

* feat: Move `RoomStatusBarHistoryVisible` to `shared-components`.

* fix: Address review comments on `RoomStatusBarHistoryVisible`.

* fix: Address review comments on `RoomStatusBar` and tests.

* chore: Move `RoomStatusBarHistoryVisible` to `room/RoomStatusBarHistoryVisible`

* chore: Fix linting issues.

* feat: Gate behind history visibility labs flag.

* feat: Add link to history sharing docs.

* fix: Resolve build issue with shared-components.

* tests: Enable history sharing lab for unit tests.

* tests: Set labs flag in SettingsStore mock.

* fix: Remove non-existent arg - documentation should be updated!

* chore: Remove old CSS rule filter.

* fix: Use package name for import over relative path.

* fix: Mark styles as important due to improper CSS load order.

* docs: Add doc comments to `!important` directives.

This change should restore my status as a good person.

* docs: Correct license header.

* tests: Update `RoomStatusBarHistoryVisible` snapshot.

* tests: Update shared history invite screenshot.

* tests: Revert spurious screenshot changes.

* feat: Update to use `Banner` component.

* chore: Remove broken import.

* chore: Remove unused translation string.

* tests: Add `getHistoryVisibility` to `currentState` of stub room.

* tests: Update screenshot.

* chore: Remove old snapshots.

* tests: Update playwright screenshot.

* feat: Separate `HistoryVisibleBanner` hooks into MVVM architecture.

* chore: Remove unused imports.

* feat: Use info link over action button for `HistoryVisibleBanner`

* tests: Update snapshot for `HistoryVisibleBanner`.

* chore: Remove unused imports.

* feat: Switch to MVVM architecture per style guide.

* tests: Update snapshot for `HistoryVisibleBanner`.

* tests: Update shared components snapshots.

* tests: Add unit tests for `HistoryVisibleBannerView` stories.

* fix: Linting errors from SonarCloud.

* feat: Finalise conversion to MVVM.

* fix: Silent `this` binding issue.

* tests: Update playwright snapshot.

* feat: Introduce wrapper component for `HistoryVisibleBanner`.

* tests: Update playwright screenshots for `HistoryVisibleBanner`.

* docs: Add doc comments to fields in `HistoryVisibleBannerViewModel`.

* tests: Update playwright snapshot.

src/components/views/composer/HistoryVisibleBanner.tsx

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2025 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import { HistoryVisibleBannerView, useCreateAutoDisposedViewModel } from "@element-hq/web-shared-components";
+import React from "react";
+import { type Room } from "matrix-js-sdk/src/matrix";
+
+import { HistoryVisibleBannerViewModel } from "../../../viewmodels/composer/HistoryVisibleBannerViewModel";
+
+export const HistoryVisibleBanner: React.FC<{ room: Room }> = ({ room }) => {
+    const vm = useCreateAutoDisposedViewModel(() => new HistoryVisibleBannerViewModel({ room }));
+    return <HistoryVisibleBannerView vm={vm} />;
+};

src/components/views/rooms/MessageComposer.tsx

@@ -54,6 +54,7 @@ import { type MatrixClientProps, withMatrixClientHOC } from "../../../contexts/M
 import { UIFeature } from "../../../settings/UIFeature";
 import { formatTimeLeft } from "../../../DateUtils";
 import RoomReplacedSvg from "../../../../res/img/room_replaced.svg";
+import { HistoryVisibleBanner } from "../composer/HistoryVisibleBanner";
 
 // The prefix used when persisting editor drafts to localstorage.
 export const WYSIWYG_EDITOR_STATE_STORAGE_PREFIX = "mx_wysiwyg_state_";
@@ -674,6 +675,7 @@ export class MessageComposer extends React.Component<IProps, IState> {
 
         return (
             <div className={classes} ref={this.ref} role="region" aria-label={_t("a11y|message_composer")}>
+                <HistoryVisibleBanner room={this.props.room} />
                 <div className="mx_MessageComposer_wrapper">
                     <UserIdentityWarning room={this.props.room} key={this.props.room.roomId} />
                     <ReplyPreview

src/i18n/strings/en_EN.json

@@ -2123,6 +2123,7 @@
         "status_bar": {
             "delete_all": "Delete all",
             "exceeded_resource_limit": "Your message wasn't sent because this homeserver has exceeded a resource limit. Please <a>contact your service administrator</a> to continue using the service.",
+            "history_visible": "Messages you send will be shared with new members invited to this room. <a>Learn more</a>",
             "homeserver_blocked": "Your message wasn't sent because this homeserver has been blocked by its administrator. Please <a>contact your service administrator</a> to continue using the service.",
             "monthly_user_limit_reached": "Your message wasn't sent because this homeserver has hit its Monthly Active User Limit. Please <a>contact your service administrator</a> to continue using the service.",
             "requires_consent_agreement": "You can't send any messages until you review and agree to <consentLink>our terms and conditions</consentLink>.",

src/settings/Settings.tsx

@@ -371,6 +371,7 @@ export interface Settings {
     "inviteRules": IBaseSetting<ComputedInviteConfig>;
     "blockInvites": IBaseSetting<boolean>;
     "Developer.elementCallUrl": IBaseSetting<string>;
+    "acknowledgedHistoryVisibility": IBaseSetting<boolean>;
 }
 
 export type SettingKey = keyof Settings;
@@ -1488,4 +1489,8 @@ export const SETTINGS: Settings = {
         displayName: _td("devtools|settings|elementCallUrl"),
         default: "",
     },
+    "acknowledgedHistoryVisibility": {
+        supportedLevels: [SettingLevel.ROOM_ACCOUNT],
+        default: false,
+    },
 };

src/viewmodels/composer/HistoryVisibleBannerViewModel.tsx

@@ -0,0 +1,112 @@
+/*
+ * Copyright (c) 2025 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import {
+    BaseViewModel,
+    type HistoryVisibleBannerViewModel as HistoryVisibleBannerViewModelInterface,
+    type HistoryVisibleBannerViewSnapshot,
+} from "@element-hq/web-shared-components";
+import { HistoryVisibility, RoomStateEvent, type Room } from "matrix-js-sdk/src/matrix";
+
+import SettingsStore from "../../settings/SettingsStore";
+import { SettingLevel } from "../../settings/SettingLevel";
+
+interface Props {
+    room: Room;
+}
+
+export class HistoryVisibleBannerViewModel
+    extends BaseViewModel<HistoryVisibleBannerViewSnapshot, Props>
+    implements HistoryVisibleBannerViewModelInterface
+{
+    /**
+     * Watcher ID for the "feature_share_history_on_invite" setting.
+     */
+    private readonly featureWatcher: string;
+
+    /**
+     * Watcher ID for the "acknowledgedHistoryVisibility" setting specific to the room.
+     */
+    private readonly acknowledgedWatcher: string;
+
+    private static readonly computeSnapshot = (room: Room): HistoryVisibleBannerViewSnapshot => {
+        const featureEnabled = SettingsStore.getValue("feature_share_history_on_invite");
+        const acknowledged = SettingsStore.getValue("acknowledgedHistoryVisibility", room.roomId);
+
+        return {
+            visible:
+                featureEnabled &&
+                room.hasEncryptionStateEvent() &&
+                room.getHistoryVisibility() !== HistoryVisibility.Joined &&
+                !acknowledged,
+        };
+    };
+
+    public constructor(props: Props) {
+        super(props, HistoryVisibleBannerViewModel.computeSnapshot(props.room));
+
+        this.disposables.trackListener(props.room, RoomStateEvent.Update, () => this.setSnapshot());
+
+        // `SettingsStore` is not an `EventListener`, so we must manage these manually.
+        this.featureWatcher = SettingsStore.watchSetting(
+            "feature_share_history_on_invite",
+            null,
+            (_key, _roomId, _level, value: boolean) => this.setSnapshot(),
+        );
+        this.acknowledgedWatcher = SettingsStore.watchSetting(
+            "acknowledgedHistoryVisibility",
+            props.room.roomId,
+            (_key, _roomId, _level, value: boolean) => this.setSnapshot(),
+        );
+    }
+
+    private setSnapshot(): void {
+        const acknowledged = SettingsStore.getValue("acknowledgedHistoryVisibility", this.props.room.roomId);
+
+        // Reset the acknowleded flag when the history visibility is set back to joined.
+        if (this.props.room.getHistoryVisibility() === HistoryVisibility.Joined && acknowledged) {
+            SettingsStore.setValue(
+                "acknowledgedHistoryVisibility",
+                this.props.room.roomId,
+                SettingLevel.ROOM_ACCOUNT,
+                false,
+            );
+        }
+
+        this.snapshot.set(HistoryVisibleBannerViewModel.computeSnapshot(this.props.room));
+    }
+
+    /**
+     * Revoke the banner's acknoledgement status.
+     */
+    public async revoke(): Promise<void> {
+        await SettingsStore.setValue(
+            "acknowledgedHistoryVisibility",
+            this.props.room.roomId,
+            SettingLevel.ROOM_ACCOUNT,
+            false,
+        );
+    }
+
+    /**
+     * Called when the user dismisses the banner.
+     */
+    public async onClose(): Promise<void> {
+        await SettingsStore.setValue(
+            "acknowledgedHistoryVisibility",
+            this.props.room.roomId,
+            SettingLevel.ROOM_ACCOUNT,
+            true,
+        );
+    }
+
+    public dispose(): void {
+        super.dispose();
+        SettingsStore.unwatchSetting(this.featureWatcher);
+        SettingsStore.unwatchSetting(this.acknowledgedWatcher);
+    }
+}