Partially viewable bottom sheet - flutter

Use the DraggableScrollableSheet widget with the Stack widget:

Here's the gist for the entire page in this^ GIF, or try the Codepen.

Here's the structure of the entire page:

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: <Widget>[
          CustomGoogleMap(),
          CustomHeader(),
          DraggableScrollableSheet(
            initialChildSize: 0.30,
            minChildSize: 0.15,
            builder: (BuildContext context, ScrollController scrollController) {
              return SingleChildScrollView(
                controller: scrollController,
                child: CustomScrollViewContent(),
              );
            },
          ),
        ],
      ),
    );
  }

In the Stack:
- The Google map is the lower most layer.
- The Header (search field + horizontally scrolling chips) is above the map.
- The DraggableBottomSheet is above the Header.

Some useful parameters as defined in draggable_scrollable_sheet.dart:

  /// The initial fractional value of the parent container's height to use when
  /// displaying the widget.
  ///
  /// The default value is `0.5`.
  final double initialChildSize;

  /// The minimum fractional value of the parent container's height to use when
  /// displaying the widget.
  ///
  /// The default value is `0.25`.
  final double minChildSize;

  /// The maximum fractional value of the parent container's height to use when
  /// displaying the widget.
  ///
  /// The default value is `1.0`.
  final double maxChildSize;

Edit: Thank you @Alejandro for pointing out the typo in the widget name :)

I will be implementing the same behaviour in the next few weeks and I will be referring to the backdrop implementation in Flutter Gallery, I was able to modify it previously to swipe to display and hide (with a peek area).

To be precise you can replicate the desired effect by changing this line of code in backdrop_demo.dart from Flutter Gallery :

 void _handleDragUpdate(DragUpdateDetails details) {
    if (_controller.isAnimating)// || _controller.status == AnimationStatus.completed)
      return;

    _controller.value -= details.primaryDelta / (_backdropHeight ?? details.primaryDelta);
 }

I have just commented the controller status check to allow the panel to be swipe-able.

I know this isn't the complete implementation you are looking for, but I hope this helps you in any way.