Preserve composer focus when adding thread posts (#11605)

src/view/com/composer/Composer.tsx

@@ -1700,7 +1700,7 @@ let ComposerPost = memo(function ComposerPost({
           style={[a.pt_xs]}
           richtext={richtext}
           placeholder={selectTextInputPlaceholder}
-          autoFocus={isLastPost}
+          autoFocus={isActive}
           webForceMinHeight={forceMinHeight}
           // To avoid overlap with the close button:
           hasRightPadding={isPartOfThread}

src/view/com/composer/state/composer.test.ts

@@ -0,0 +1,60 @@
+import {composerReducer, createComposerState} from './composer'
+
+jest.mock('#/state/gallery', () => ({
+  createInitialImages: jest.fn(),
+}))
+jest.mock('#/logger', () => ({
+  logger: {
+    warn: jest.fn(),
+  },
+}))
+jest.mock('#/state/queries/postgate/util', () => ({
+  createPostgateRecord: jest.fn(() => ({})),
+}))
+jest.mock('#/state/queries/threadgate', () => ({
+  threadgateRecordToAllowUISetting: jest.fn(() => []),
+}))
+
+function createState() {
+  return createComposerState({
+    initText: undefined,
+    initMention: undefined,
+    initImageUris: undefined,
+    initQuoteUri: undefined,
+    initInteractionSettings: undefined,
+  })
+}
+
+describe('composerReducer', () => {
+  describe('add_post', () => {
+    it('selects the appended post and requests focus', () => {
+      const state = createState()
+
+      const nextState = composerReducer(state, {type: 'add_post'})
+
+      expect(nextState.thread.posts).toHaveLength(2)
+      expect(nextState.activePostIndex).toBe(1)
+      expect(nextState.mutableNeedsFocusActive).toBe(true)
+    })
+
+    it('selects a post inserted in the middle of a thread', () => {
+      let state = createState()
+      state = composerReducer(state, {type: 'add_post'})
+      state = composerReducer(state, {type: 'add_post'})
+
+      const lastPostId = state.thread.posts[2].id
+      state = composerReducer(state, {
+        type: 'focus_post',
+        postId: state.thread.posts[0].id,
+      })
+
+      const nextState = composerReducer(state, {type: 'add_post'})
+
+      expect(nextState.thread.posts).toHaveLength(4)
+      expect(nextState.activePostIndex).toBe(1)
+      expect(nextState.thread.posts[2].id).not.toBe(lastPostId)
+      expect(nextState.thread.posts[3].id).toBe(lastPostId)
+      expect(nextState.mutableNeedsFocusActive).toBe(true)
+    })
+  })
+})

src/view/com/composer/state/composer.ts

@@ -253,6 +253,8 @@ export function composerReducer(
       return {
         ...state,
         isDirty: true,
+        activePostIndex: activePostIndex + 1,
+        mutableNeedsFocusActive: true,
         thread: {
           ...state.thread,
           posts: nextPosts,