Как создать кнопку L-образной формы во Flutter?

Вы можете комбинировать Stack и ShapeBorder для кнопки L.

class LShapeBorder extends OutlinedBorder {
  @override
  OutlinedBorder copyWith({BorderSide? side}) => this;

  @override
  ui.Path getInnerPath(ui.Rect rect, {ui.TextDirection? textDirection}) =>
      getOuterPath(rect, textDirection: textDirection);

  @override
  ui.Path getOuterPath(ui.Rect rect, {ui.TextDirection? textDirection}) {
    // L shape
    final Path path = Path();

    final btnW = rect.width / 2 - 10;
    // path.moveTo(rect.right - btnW, rect.top);
    path.addRRect(
      RRect.fromRectAndRadius(
        Rect.fromLTRB(rect.right - btnW, rect.top, rect.right, rect.bottom),
        const Radius.circular(8),
      ),
    );

    path.addRRect(
      RRect.fromRectAndRadius(
        Rect.fromLTRB(rect.left, rect.bottom - btnW, rect.right, rect.bottom),
        const Radius.circular(8),
      ),
    );

    path.close();
    return path;
  }

  @override
  void paint(ui.Canvas canvas, ui.Rect rect,
      {ui.TextDirection? textDirection}) {}

  @override
  ShapeBorder scale(double t) => this;
}

И вариант использования

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: SizedBox(
            height: 200,
            width: 200,
            child: Stack(
              children: [
                Align(
                  alignment: Alignment.topLeft,
                  child: ElevatedButton(
                    style: ElevatedButton.styleFrom(
                        fixedSize: Size.square(100),
                        backgroundColor: Colors.red,
                        shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(8),
                        )),
                    onPressed: () {
                      debugPrint('1');
                    },
                    child: SizedBox(),
                  ),
                ),
                Positioned.fill(
                  child: ElevatedButton(
                    style: ElevatedButton.styleFrom(
                      backgroundColor: Colors.blue,
                      shape: LShapeBorder(),
                    ),
                    onPressed: () {
                      debugPrint('2');
                    },
                    child: SizedBox(),
                  ),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}