ShowsDebugPaint widget - paint outlines for a single widget and its child subtree - by Simon Lightfoot - 13/05/2020 (Not a fix for Flutter Web debug paint issue.. https://github.com/flutter/flutter/issues/32221 )

show_debug_paint.dart

// MIT License
//
// Copyright (c) 2020 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/cupertino.dart';
import 'package:flutter/rendering.dart';

class ShowDebugPaint extends SingleChildRenderObjectWidget {
  const ShowDebugPaint({
    Key? key,
    this.enabled = true,
    required Widget child,
  }) : super(key: key, child: child);

  final bool enabled;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return RenderShowDebugPaint(enabled: enabled);
  }

  @override
  void updateRenderObject(BuildContext context, RenderShowDebugPaint renderObject) {
    renderObject.enabled = enabled;
  }
}

class RenderShowDebugPaint extends RenderProxyBox {
  RenderShowDebugPaint({
    required bool enabled,
    RenderBox? child,
  })  : _enabled = enabled,
        super(child);

  bool _enabled;

  bool get enabled => _enabled;

  set enabled(bool value) {
    if (_enabled != value) {
      _enabled = value;
      markNeedsPaint();
    }
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final previousState = debugPaintSizeEnabled;
    debugPaintSizeEnabled = enabled;
    super.paint(context, offset);
    debugPaintSizeEnabled = previousState;
  }
}