Fix unlucky failures in character replacement

Summary:
Fixes #1830 (bug introduced in #719; this is a proper fix for #398).

* When we're allowing native insertion, we don't want to modify the selection because doing so blocks browser features like spellcheck from working
* In all other cases when typing, we *do* want to force the selection – failing to do this is why the original issue occurred
* We use the `insert-characters` change type for both native and non-native insertions
* Instead of guessing in `EditorState.push` whether to force a selection, accept it as a parameter
* Gate all changes by `draft_non_native_insertion_forces_selection`

I verified with the feature flag disabled (same behavior as before):

* Typing over a character works, and the cursor moves as expected (because of the `chars === currentlySelectedChars` check)
* With editor contents "ABC\ndef", replacing "d", "e", or "f" with the character above *fails* to replace the char (because that check is buggy).

And with the feature flag enabled (new behavior):

* Typing over a character still works (but does so without the buggy check)
* With editor contents "ABC\ndef", replacing "d", "e", or "f" with the character above *succeeds* at replacing the char.

Reviewed By: flarnie

Differential Revision: D9209691

fbshipit-source-id: f63551dbad689391aa9c2a69f1d6ba395b8bf1ac

Author: sophiebits

src/component/handlers/edit/editOnBeforeInput.js

@@ -20,6 +20,7 @@ const EditorState = require('EditorState');
 const UserAgent = require('UserAgent');
 
 const getEntityKeyForSelection = require('getEntityKeyForSelection');
+const gkx = require('gkx');
 const isEventHandled = require('isEventHandled');
 const isSelectionAtLeafStart = require('isSelectionAtLeafStart');
 const nullthrows = require('nullthrows');
@@ -36,6 +37,10 @@ const FF_QUICKFIND_CHAR = "'";
 const FF_QUICKFIND_LINK_CHAR = '/';
 const isFirefox = UserAgent.isBrowser('Firefox');
 
+const nonNativeInsertionForcesSelection = gkx(
+  'draft_non_native_insertion_forces_selection',
+);
+
 function mustPreventDefaultForCharacter(character: string): boolean {
   return (
     isFirefox &&
@@ -52,6 +57,7 @@ function replaceText(
   text: string,
   inlineStyle: DraftInlineStyle,
   entityKey: ?string,
+  forceSelection: boolean,
 ): EditorState {
   const contentState = DraftModifier.replaceText(
     editorState.getCurrentContent(),
@@ -60,7 +66,12 @@ function replaceText(
     inlineStyle,
     entityKey,
   );
-  return EditorState.push(editorState, contentState, 'insert-characters');
+  return EditorState.push(
+    editorState,
+    contentState,
+    'insert-characters',
+    forceSelection,
+  );
 }
 
 /**
@@ -124,7 +135,10 @@ function editOnBeforeInput(
       .getCurrentContent()
       .getPlainText()
       .slice(selectionStart, selectionEnd);
-    if (chars === currentlySelectedChars) {
+    if (
+      !nonNativeInsertionForcesSelection &&
+      chars === currentlySelectedChars
+    ) {
       editor.update(
         EditorState.forceSelection(
           editorState,
@@ -144,6 +158,7 @@ function editOnBeforeInput(
             editorState.getCurrentContent(),
             editorState.getSelection(),
           ),
+          true,
         ),
       );
     }
@@ -158,6 +173,7 @@ function editOnBeforeInput(
       editorState.getCurrentContent(),
       editorState.getSelection(),
     ),
+    false,
   );
 
   // Bunch of different cases follow where we need to prevent native insertion.
@@ -254,6 +270,11 @@ function editOnBeforeInput(
 
   if (mustPreventNative) {
     e.preventDefault();
+    if (nonNativeInsertionForcesSelection) {
+      newEditorState = EditorState.set(newEditorState, {
+        forceSelection: true,
+      });
+    }
     editor.update(newEditorState);
     return;
   }

src/model/immutable/EditorState.js

@@ -22,9 +22,11 @@ import type {List, OrderedMap} from 'immutable';
 const BlockTree = require('BlockTree');
 const ContentState = require('ContentState');
 const EditorBidiService = require('EditorBidiService');
-const Immutable = require('immutable');
 const SelectionState = require('SelectionState');
 
+const gkx = require('gkx');
+const Immutable = require('immutable');
+
 const {OrderedSet, Record, Stack} = Immutable;
 
 type EditorStateRecordType = {
@@ -340,12 +342,15 @@ class EditorState {
     editorState: EditorState,
     contentState: ContentState,
     changeType: EditorChangeType,
+    forceSelection: boolean = true,
   ): EditorState {
     if (editorState.getCurrentContent() === contentState) {
       return editorState;
     }
 
-    const forceSelection = changeType !== 'insert-characters';
+    if (!gkx('draft_non_native_insertion_forces_selection')) {
+      forceSelection = changeType !== 'insert-characters';
+    }
     const directionMap = EditorBidiService.getDirectionMap(
       contentState,
       editorState.getDirectionMap(),