Update CompositionAwareMixin to correctly compute composingBase in Web engine (flutter#44139)

It fixes an issue in `CompositionAwareMixin` causing wrong deltas being computed and reported to input clients resulting in wrong state when compositing text in rich text editors like [Fleather](https://github.com/fleather-editor/fleather) and possibly others.

Reported deltas on Mac:
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ��
TextEditingDeltaInsertion: start: 2, end: 0, data: �
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ����
TextEditingDeltaReplacement: start: 0, end: 4, data: ���
```

Reported deltas on Web (Before):
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ��
TextEditingDeltaInsertion: start: 2, end: 0, data: �
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ����
TextEditingDeltaInsertion: start: 4, end: 0, data: ���
```

Reported deltas on Web (After):
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ��
TextEditingDeltaInsertion: start: 2, end: 0, data: �
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ����
TextEditingDeltaReplacement: start: 0, end: 4, data: ���
```

* Fixes flutter/flutter#131335
* Fixes fleather-editor/fleather#150

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: Amir-P

AUTHORS

@@ -23,4 +23,5 @@ Koutaro Mori <[email protected]>
 TheOneWithTheBraid <[email protected]>
 Twin Sun, LLC <[email protected]>
 Qixing Cao <[email protected]>
-LinXunFeng <[email protected]>
+LinXunFeng <[email protected]>
+Amir Panahandeh <[email protected]>

lib/web_ui/lib/src/engine/text_editing/composition_aware_mixin.dart

@@ -71,11 +71,11 @@ mixin CompositionAwareMixin {
   }
 
   EditingState determineCompositionState(EditingState editingState) {
-    if (editingState.baseOffset == null || composingText == null || editingState.text == null) {
+    if (editingState.extentOffset == null || composingText == null || editingState.text == null) {
       return editingState;
     }
 
-    final int composingBase = editingState.baseOffset! - composingText!.length;
+    final int composingBase = editingState.extentOffset! - composingText!.length;
 
     if (composingBase < 0) {
       return editingState;

lib/web_ui/test/engine/composition_test.dart

@@ -106,7 +106,7 @@ Future<void> testMain() async {
     });
 
     group('determine composition state', () {
-      test('should return new composition state if valid new composition', () {
+      test('should return new composition state - compositing middle of text', () {
         const int baseOffset = 100;
         const String composingText = 'composeMe';
 
@@ -128,6 +128,30 @@ Future<void> testMain() async {
                 composingBaseOffset: expectedComposingBase,
                 composingExtentOffset: expectedComposingBase + composingText.length));
       });
+
+      test('should return new composition state - compositing from beginning of text', () {
+        const String composingText = '今日は';
+
+        final EditingState editingState = EditingState(
+          text: '今日は',
+          baseOffset: 0,
+          extentOffset: 3,
+        );
+
+        final _MockWithCompositionAwareMixin mockWithCompositionAwareMixin =
+            _MockWithCompositionAwareMixin();
+        mockWithCompositionAwareMixin.composingText = composingText;
+
+        const int expectedComposingBase = 0;
+
+        expect(
+            mockWithCompositionAwareMixin
+                .determineCompositionState(editingState),
+            editingState.copyWith(
+                composingBaseOffset: expectedComposingBase,
+                composingExtentOffset:
+                    expectedComposingBase + composingText.length));
+      });
     });
   });