Handle selection ranges where getBoxesForSelection returns an empty list (#59014)

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

@@ -1462,13 +1462,15 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
 
     final Offset paintOffset = _paintOffset;
 
-    if (selection.isCollapsed) {
+
+    final List<ui.TextBox> boxes = selection.isCollapsed ?
+        <ui.TextBox>[] : _textPainter.getBoxesForSelection(selection);
+    if (boxes.isEmpty) {
       // TODO(mpcomplete): This doesn't work well at an RTL/LTR boundary.
       final Offset caretOffset = _textPainter.getOffsetForCaret(selection.extent, _caretPrototype);
       final Offset start = Offset(0.0, preferredLineHeight) + caretOffset + paintOffset;
       return <TextSelectionPoint>[TextSelectionPoint(start, null)];
     } else {
-      final List<ui.TextBox> boxes = _textPainter.getBoxesForSelection(selection);
       final Offset start = Offset(boxes.first.start, boxes.first.bottom) + paintOffset;
       final Offset end = Offset(boxes.last.end, boxes.last.bottom) + paintOffset;
       return <TextSelectionPoint>[

packages/flutter/test/rendering/editable_test.dart

@@ -796,4 +796,23 @@ void main() {
     await simulateKeyUpEvent(LogicalKeyboardKey.delete, platform: 'android');
     expect(delegate.textEditingValue.text, '');
   }, skip: kIsWeb); // Key simulation doesn't work on web.
+
+  test('getEndpointsForSelection handles empty characters', () {
+    final TextSelectionDelegate delegate = FakeEditableTextState();
+    final RenderEditable editable = RenderEditable(
+      // This is a Unicode left-to-right mark character that will not render
+      // any glyphs.
+      text: const TextSpan(text: '\u200e'),
+      textAlign: TextAlign.start,
+      textDirection: TextDirection.ltr,
+      offset: ViewportOffset.zero(),
+      textSelectionDelegate: delegate,
+      startHandleLayerLink: LayerLink(),
+      endHandleLayerLink: LayerLink(),
+    );
+    editable.layout(BoxConstraints.loose(const Size(100, 100)));
+    final List<TextSelectionPoint> endpoints = editable.getEndpointsForSelection(
+      const TextSelection(baseOffset: 0, extentOffset: 1));
+    expect(endpoints[0].point.dx, 0);
+  });
 }