Gracefully handle negative position in getWordAtOffset (#128464)

Should fix an unreproducible crash in text editing and track it with an assertion.

packages/flutter/lib/src/rendering/editable.dart

@@ -2067,9 +2067,9 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
   void selectWordsInRange({ required Offset from, Offset? to, required SelectionChangedCause cause }) {
     _computeTextMetricsIfNeeded();
     final TextPosition fromPosition = _textPainter.getPositionForOffset(globalToLocal(from - _paintOffset));
-    final TextSelection fromWord = _getWordAtOffset(fromPosition);
+    final TextSelection fromWord = getWordAtOffset(fromPosition);
     final TextPosition toPosition = to == null ? fromPosition : _textPainter.getPositionForOffset(globalToLocal(to - _paintOffset));
-    final TextSelection toWord = toPosition == fromPosition ? fromWord : _getWordAtOffset(toPosition);
+    final TextSelection toWord = toPosition == fromPosition ? fromWord : getWordAtOffset(toPosition);
     final bool isFromWordBeforeToWord = fromWord.start < toWord.end;
 
     _setSelection(
@@ -2099,7 +2099,10 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     _setSelection(newSelection, cause);
   }
 
-  TextSelection _getWordAtOffset(TextPosition position) {
+  /// Returns a [TextSelection] that encompasses the word at the given
+  /// [TextPosition].
+  @visibleForTesting
+  TextSelection getWordAtOffset(TextPosition position) {
     debugAssertLayoutUpToDate();
     // When long-pressing past the end of the text, we want a collapsed cursor.
     if (position.offset >= plainText.length) {
@@ -2120,6 +2123,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
       case TextAffinity.downstream:
         effectiveOffset = position.offset;
     }
+    assert(effectiveOffset >= 0);
 
     // On iOS, select the previous word if there is a previous word, or select
     // to the end of the next word if there is a next word. Select nothing if
@@ -2128,8 +2132,8 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     // If the platform is Android and the text is read only, try to select the
     // previous word if there is one; otherwise, select the single whitespace at
     // the position.
-    if (TextLayoutMetrics.isWhitespace(plainText.codeUnitAt(effectiveOffset))
-        && effectiveOffset > 0) {
+    if (effectiveOffset > 0
+        && TextLayoutMetrics.isWhitespace(plainText.codeUnitAt(effectiveOffset))) {
       final TextRange? previousWord = _getPreviousWord(word.start);
       switch (defaultTargetPlatform) {
         case TargetPlatform.iOS:

packages/flutter/test/rendering/editable_test.dart

@@ -1823,6 +1823,52 @@ void main() {
       rrect: expectedRRect
     ));
   });
+
+  test('getWordAtOffset with a negative position', () {
+    const String text = 'abc';
+    final _FakeEditableTextState delegate = _FakeEditableTextState()
+      ..textEditingValue = const TextEditingValue(text: text);
+    final ViewportOffset viewportOffset = ViewportOffset.zero();
+    final RenderEditable editable = RenderEditable(
+      backgroundCursorColor: Colors.grey,
+      selectionColor: Colors.black,
+      textDirection: TextDirection.ltr,
+      cursorColor: Colors.red,
+      offset: viewportOffset,
+      textSelectionDelegate: delegate,
+      startHandleLayerLink: LayerLink(),
+      endHandleLayerLink: LayerLink(),
+      text: const TextSpan(
+        text: text,
+        style: TextStyle(height: 1.0, fontSize: 10.0),
+      ),
+    );
+
+    layout(editable, onErrors: expectNoFlutterErrors);
+
+    // Cause text metrics to be computed.
+    editable.computeDistanceToActualBaseline(TextBaseline.alphabetic);
+
+    final TextSelection selection;
+    try {
+      selection = editable.getWordAtOffset(const TextPosition(
+        offset: -1,
+        affinity: TextAffinity.upstream,
+      ));
+    } catch (error) {
+      // In debug mode, negative offsets are caught by an assertion.
+      expect(error, isA<AssertionError>());
+      return;
+    }
+
+    // Web's Paragraph.getWordBoundary behaves differently for a negative
+    // position.
+    if (kIsWeb) {
+      expect(selection, const TextSelection.collapsed(offset: 0));
+    } else {
+      expect(selection, const TextSelection.collapsed(offset: text.length));
+    }
+  });
 }
 
 class _TestRenderEditable extends RenderEditable {