Fix editing delta is wrong when using delete key

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

@@ -472,9 +472,12 @@ class TextEditingDeltaState {
   /// Infers the correct delta values based on information from the new editing state
   /// and the last editing state.
   ///
-  /// For a deletion we calculate the length of the deleted text by comparing the new
-  /// and last editing states. We subtract this from the [deltaEnd] that we set when beforeinput
-  /// was fired to determine the [deltaStart].
+  /// For a deletion, the length and the direction of the deletion (backward or forward)
+  /// are calculated by comparing the new and last editing states.
+  /// If the deletion is backward, the length is susbtracted from the [deltaEnd]
+  /// that we set when beforeinput was fired to determine the [deltaStart].
+  /// If the deletion is forward, [deltaStart] is set to the new editing state baseOffset
+  /// and [deltaEnd] is set to [deltaStart] incremented by the length of the deletion.
   ///
   /// For a replacement at a selection we set the [deltaStart] to be the beginning of the selection
   /// from the last editing state.
@@ -495,9 +498,19 @@ class TextEditingDeltaState {
     if (isTextBeingRemoved) {
       // When text is deleted outside of the composing region or is cut using the native toolbar,
       // we calculate the length of the deleted text by comparing the new and old editing state lengths.
-      // This value is then subtracted from the end position of the delta to capture the deleted range.
+      // If the deletion is backward, the length is susbtracted from the [deltaEnd]
+      // that we set when beforeinput was fired to determine the [deltaStart].
+      // If the deletion is forward, [deltaStart] is set to the new editing state baseOffset
+      // and [deltaEnd] is set to [deltaStart] incremented by the length of the deletion.
       final int deletedLength = newTextEditingDeltaState.oldText.length - newEditingState.text!.length;
-      newTextEditingDeltaState.deltaStart = newTextEditingDeltaState.deltaEnd - deletedLength;
+      final bool backwardDeletion = newEditingState.baseOffset != lastEditingState?.baseOffset;
+      if (backwardDeletion) {
+        newTextEditingDeltaState.deltaStart = newTextEditingDeltaState.deltaEnd - deletedLength;
+      } else {
+        // Forward deletion
+        newTextEditingDeltaState.deltaStart = newEditingState.baseOffset!;
+        newTextEditingDeltaState.deltaEnd = newTextEditingDeltaState.deltaStart + deletedLength;
+      }
     } else if (isTextBeingChangedAtActiveSelection) {
       // When a selection of text is replaced by a copy/paste operation we set the starting range
       // of the delta to be the beginning of the selection of the previous editing state.

lib/web_ui/test/text_editing_test.dart

@@ -2349,10 +2349,18 @@ Future<void> testMain() async {
       expect(textEditingDeltaState.composingExtent, -1);
     });
 
-    test('Verify correct delta is inferred - deletion', () {
+    test('Verify correct delta is inferred - Backward deletion - Empty selection', () {
       final EditingState newEditState = EditingState(text: 'worl', baseOffset: 4, extentOffset: 4);
       final EditingState lastEditState = EditingState(text: 'world', baseOffset: 5, extentOffset: 5);
-      final TextEditingDeltaState deltaState = TextEditingDeltaState(oldText: 'world', deltaStart: 4, deltaEnd: 5, baseOffset: -1, extentOffset: -1, composingOffset: -1, composingExtent: -1);
+      // `deltaState.deltaEnd` is initialized accordingly to what is done in `DefaultTextEditingStrategy.handleBeforeInput`
+      final TextEditingDeltaState deltaState = TextEditingDeltaState(
+        oldText: 'world',
+        deltaEnd: 5,
+        baseOffset: -1,
+        extentOffset: -1,
+        composingOffset: -1,
+        composingExtent: -1,
+      );
 
       final TextEditingDeltaState textEditingDeltaState = TextEditingDeltaState.inferDeltaState(newEditState, lastEditState, deltaState);
 
@@ -2366,6 +2374,56 @@ Future<void> testMain() async {
       expect(textEditingDeltaState.composingExtent, -1);
     });
 
+    test('Verify correct delta is inferred - Forward deletion - Empty selection', () {
+      final EditingState newEditState = EditingState(text: 'worl', baseOffset: 4, extentOffset: 4);
+      final EditingState lastEditState = EditingState(text: 'world', baseOffset: 4, extentOffset: 4);
+      // `deltaState.deltaEnd` is initialized accordingly to what is done in `DefaultTextEditingStrategy.handleBeforeInput`
+      final TextEditingDeltaState deltaState = TextEditingDeltaState(
+        oldText: 'world',
+        deltaEnd: 4,
+        baseOffset: -1,
+        extentOffset: -1,
+        composingOffset: -1,
+        composingExtent: -1,
+      );
+
+      final TextEditingDeltaState textEditingDeltaState = TextEditingDeltaState.inferDeltaState(newEditState, lastEditState, deltaState);
+
+      expect(textEditingDeltaState.oldText, 'world');
+      expect(textEditingDeltaState.deltaText, '');
+      expect(textEditingDeltaState.deltaStart, 4);
+      expect(textEditingDeltaState.deltaEnd, 5);
+      expect(textEditingDeltaState.baseOffset, 4);
+      expect(textEditingDeltaState.extentOffset, 4);
+      expect(textEditingDeltaState.composingOffset, -1);
+      expect(textEditingDeltaState.composingExtent, -1);
+    });
+
+    test('Verify correct delta is inferred - Deletion - Non-empty selection', () {
+      final EditingState newEditState = EditingState(text: 'w', baseOffset: 1, extentOffset: 1);
+      final EditingState lastEditState = EditingState(text: 'world', baseOffset: 1, extentOffset: 5);
+      // `deltaState.deltaEnd` is initialized accordingly to what is done in `DefaultTextEditingStrategy.handleBeforeInput`
+      final TextEditingDeltaState deltaState = TextEditingDeltaState(
+        oldText: 'world',
+        deltaEnd: 5,
+        baseOffset: -1,
+        extentOffset: -1,
+        composingOffset: -1,
+        composingExtent: -1,
+      );
+
+      final TextEditingDeltaState textEditingDeltaState = TextEditingDeltaState.inferDeltaState(newEditState, lastEditState, deltaState);
+
+      expect(textEditingDeltaState.oldText, 'world');
+      expect(textEditingDeltaState.deltaText, '');
+      expect(textEditingDeltaState.deltaStart, 1);
+      expect(textEditingDeltaState.deltaEnd, 5);
+      expect(textEditingDeltaState.baseOffset, 1);
+      expect(textEditingDeltaState.extentOffset, 1);
+      expect(textEditingDeltaState.composingOffset, -1);
+      expect(textEditingDeltaState.composingExtent, -1);
+    });
+
     test('Verify correct delta is inferred - composing region replacement', () {
       final EditingState newEditState = EditingState(text: '你好吗', baseOffset: 3, extentOffset: 3);
       final EditingState lastEditState = EditingState(text: 'ni hao ma', baseOffset: 9, extentOffset: 9);