Add "new conversation" composer for direct messages

ts/components/leftPane/LeftPaneHelper.tsx

@@ -0,0 +1,67 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { ChangeEvent, ReactChild } from 'react';
+
+import { Row } from '../ConversationList';
+import { LocalizerType } from '../../types/Util';
+
+export enum FindDirection {
+  Up,
+  Down,
+}
+
+export type ToFindType = {
+  direction: FindDirection;
+  unreadOnly: boolean;
+};
+
+/* eslint-disable class-methods-use-this */
+
+export abstract class LeftPaneHelper<T> {
+  getHeaderContents(
+    _: Readonly<{
+      i18n: LocalizerType;
+      showInbox: () => void;
+    }>
+  ): null | ReactChild {
+    return null;
+  }
+
+  shouldRenderNetworkStatusAndUpdateDialog(): boolean {
+    return false;
+  }
+
+  getPreRowsNode(
+    _: Readonly<{
+      i18n: LocalizerType;
+      onChangeComposeSearchTerm: (
+        event: ChangeEvent<HTMLInputElement>
+      ) => unknown;
+    }>
+  ): null | ReactChild {
+    return null;
+  }
+
+  abstract getRowCount(): number;
+
+  abstract getRow(rowIndex: number): undefined | Row;
+
+  getRowIndexToScrollTo(
+    _selectedConversationId: undefined | string
+  ): undefined | number {
+    return undefined;
+  }
+
+  abstract getConversationAndMessageAtIndex(
+    conversationIndex: number
+  ): undefined | { conversationId: string; messageId?: string };
+
+  abstract getConversationAndMessageInDirection(
+    toFind: Readonly<ToFindType>,
+    selectedConversationId: undefined | string,
+    selectedMessageId: undefined | string
+  ): undefined | { conversationId: string; messageId?: string };
+
+  abstract shouldRecomputeRowHeights(old: Readonly<T>): boolean;
+}