Paint a white rectangle exactly behind the text

line_text_painter.dart

class LineWidgetText extends ConsumerStatefulWidget {
  const LineWidgetText({
    required this.element,
    super.key,
  });

  final LineElement element;

  @override
  ConsumerState<LineWidgetText> createState() => _LineWidgetTextState();
}

class _LineWidgetTextState extends ConsumerState<LineWidgetText> {
  late final TextEditingController _textController;

  @override
  void initState() {
    super.initState();
    _textController = TextEditingController(
      text: widget.element.name,
    );

    _textController.addListener(() {
      setState(() {});
    });
  }

  @override
  void didUpdateWidget(covariant LineWidgetText oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (_textController.text != widget.element.name) {
      _textController.text = widget.element.name;
    }
  }

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
        builder: (context, constraint) {
          final style = context.canvas;
          final width = (constraint.maxWidth * 0.5).clamp(80, 180).toDouble();
          final size = textSize(
            _textController.text,
            style,
            maxWidth: width,
            maxLines: 3,
          );
          return Center(
            child: SizedBox(
              width: width,
              child: Stack(
                  alignment: Alignment.center,
                  children: [
                    Visibility(
                      visible: _textController.text.isNotEmpty,
                      child: Container(
                        width: size.width,
                        height: size.height,
                        color: context.background,
                      ),
                    ),
                    SizedBox(
                      width: width,
                      child: TextField(
                        style: style,
                        controller: _textController,
                        onChanged: (value) {
                          // change text in model
                        },
                        cursorColor: context.secondary,
                        decoration: const InputDecoration(
                          border: InputBorder.none,
                          contentPadding: EdgeInsets.zero,
                          focusedBorder: InputBorder.none,
                          enabledBorder: InputBorder.none,
                        ),
                        textAlign: TextAlign.center,
                        textAlignVertical: TextAlignVertical.top,
                        maxLines: 3,
                        minLines: 1,
                      ),
                    ),
                  ],
              ),
            ),
          );
        },
    );
  }
}

Size textSize(
  String text,
  TextStyle style, {
  double maxWidth = double.infinity,
  int maxLines = 1,
}) {
  final textPainter = TextPainter(
    text: TextSpan(text: text, style: style),
    maxLines: maxLines,
    textDirection: TextDirection.ltr,
  )..layout(
      maxWidth: maxWidth,
    );
  return textPainter.size;
}