Deselect message history message when topic selection changes

Fixes #93

app/src/reducers/Sidebar.ts

@@ -0,0 +1,44 @@
+import * as q from '../../../backend/src/Model'
+import { Action } from 'redux'
+import { createReducer } from './lib'
+import { Record } from 'immutable'
+
+interface SidebarModel {
+  compareMessage?: q.Message
+}
+
+const initialStateFactory = Record<SidebarModel>({
+  compareMessage: undefined,
+})
+
+export type Action = SetCompareMessage
+export enum ActionTypes {
+  SIDEBAR_SET_COMPARE_MESSAGE = 'SIDEBAR_SET_COMPARE_MESSAGE',
+  SIDEBAR_RESET_STORE = 'SIDEBAR_RESET_STORE',
+}
+
+export type SidebarState = Record<SidebarModel>
+
+const actions: {[s: string]: (state: SidebarState, action: Action) => SidebarState} = {
+  SIDEBAR_SET_COMPARE_MESSAGE: setCompareMessage,
+  SIDEBAR_RESET_STORE: resetStore,
+}
+
+export const sidebarReducer = createReducer(initialStateFactory(), actions)
+
+export interface SetCompareMessage {
+  type: ActionTypes.SIDEBAR_SET_COMPARE_MESSAGE
+  message?: q.Message
+}
+
+function setCompareMessage(state: SidebarState, action: SetCompareMessage): SidebarState {
+  return state.set('compareMessage', action.message)
+}
+
+export interface ResetStore {
+  type: ActionTypes.SIDEBAR_RESET_STORE
+}
+
+function resetStore(): SidebarState {
+  return initialStateFactory()
+}