Introduce a new design for the left pane

ts/test-both/helpers/FakeLeftPaneContainer.tsx

@@ -0,0 +1,29 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import React, { FunctionComponent } from 'react';
+import { WidthBreakpoint } from '../../components/_util';
+
+type PropsType = {
+  containerWidthBreakpoint: WidthBreakpoint;
+};
+
+const WIDTHS = {
+  [WidthBreakpoint.Wide]: 350,
+  [WidthBreakpoint.Medium]: 280,
+  [WidthBreakpoint.Narrow]: 130,
+};
+
+export const FakeLeftPaneContainer: FunctionComponent<PropsType> = ({
+  children,
+  containerWidthBreakpoint,
+}) => {
+  return (
+    <div
+      className={`module-left-pane--width-${containerWidthBreakpoint}`}
+      style={{ width: WIDTHS[containerWidthBreakpoint] }}
+    >
+      {children}
+    </div>
+  );
+};