Allow selection in composing region (#140516)

Fixes https://github.com/flutter/flutter/issues/68547 for macOS. Needs https://github.com/flutter/engine/pull/49314

packages/flutter/lib/src/widgets/editable_text.dart

@@ -218,9 +218,9 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// actions, not during the build, layout, or paint phases.
   ///
   /// This property can be set from a listener added to this
-  /// [TextEditingController]; however, one should not also set [selection]
+  /// [TextEditingController]; **however, one should not also set [selection]
   /// in a separate statement. To change both the [text] and the [selection]
-  /// change the controller's [value].
+  /// change the controller's [value].**
   set text(String newText) {
     value = value.copyWith(
       text: newText,
@@ -285,16 +285,13 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// in a separate statement. To change both the [text] and the [selection]
   /// change the controller's [value].
   ///
-  /// If the new selection is of non-zero length, or is outside the composing
-  /// range, the composing range is cleared.
+  /// If the new selection is outside the composing range, the composing range is
+  /// cleared.
   set selection(TextSelection newSelection) {
-    if (!isSelectionWithinTextBounds(newSelection)) {
+    if (!_isSelectionWithinTextBounds(newSelection)) {
       throw FlutterError('invalid text selection: $newSelection');
     }
-    final TextRange newComposing =
-        newSelection.isCollapsed && _isSelectionWithinComposingRange(newSelection)
-            ? value.composing
-            : TextRange.empty;
+    final TextRange newComposing = _isSelectionWithinComposingRange(newSelection) ? value.composing : TextRange.empty;
     value = value.copyWith(selection: newSelection, composing: newComposing);
   }
 
@@ -326,7 +323,7 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   }
 
   /// Check that the [selection] is inside of the bounds of [text].
-  bool isSelectionWithinTextBounds(TextSelection selection) {
+  bool _isSelectionWithinTextBounds(TextSelection selection) {
     return selection.start <= text.length && selection.end <= text.length;
   }
 
@@ -3650,7 +3647,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     // We return early if the selection is not valid. This can happen when the
     // text of [EditableText] is updated at the same time as the selection is
     // changed by a gesture event.
-    if (!widget.controller.isSelectionWithinTextBounds(selection)) {
+    if (!widget.controller._isSelectionWithinTextBounds(selection)) {
       return;
     }
 

packages/flutter/test/widgets/editable_text_test.dart

@@ -11185,14 +11185,15 @@ void main() {
     ));
     expect(state.currentTextEditingValue.composing, const TextRange(start: 4, end: 12));
 
-    // Setting a selection within the composing range clears the composing range.
+    // Setting a selection within the composing range doesn't clear the composing range.
     state.updateEditingValue(const TextEditingValue(
       text: 'foo composing bar',
       selection: TextSelection.collapsed(offset: 4),
       composing: TextRange(start: 4, end: 12),
     ));
     controller.selection = const TextSelection(baseOffset: 5, extentOffset: 7);
-    expect(state.currentTextEditingValue.composing, TextRange.empty);
+    expect(state.currentTextEditingValue.composing, const TextRange(start: 4, end: 12));
+    expect(state.currentTextEditingValue.selection, const TextSelection(baseOffset: 5, extentOffset: 7));
 
     // Reset the composing range.
     state.updateEditingValue(const TextEditingValue(