Default Scrollbars on Desktop

Summary

ScrollBehaviors now automatically apply Scrollbars to scrolling widgets on desktop platforms - Mac, Windows and Linux.

Context

Prior to this change, Scrollbars were applied to scrolling widgets manually by the developer across all platforms. This did not match developer expectations when executing Flutter applications on desktop platforms.

Now, the inherited ScrollBehavior applies a Scrollbar automatically to most scrolling widgets. This is similar to how GlowingOverscrollIndicator is created by ScrollBehavior. The few widgets that are exempt from this behavior are listed below.

To provide better management and control of this feature, ScrollBehavior has also been updated. The buildViewportChrome method, which applied a GlowingOverscrollIndicator, has been deprecated. Instead, ScrollBehavior now supports individual methods for decorating the viewport, buildScrollbar and buildOverscrollIndicator. These methods can be overridden to control what is built around the scrollable.

Further more, ScrollBehavior subclasses MaterialScrollBehavior and CupertinoScrollBehavior have been made public, allowing developers to extend and build upon the other existing ScrollBehaviors in the framework. These subclasses were previously private.

Description of change

The previous approach called on developers to create their own Scrollbars on all platforms. In some use cases, a ScrollController would need to be provided to the Scrollbar and the scrollable widget.

final ScrollController controller = ScrollController();
Scrollbar(
  controller: controller,
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
      return Text('Item $index');
    }
  )
);

The ScrollBehavior now applies the Scrollbar automatically when executing on desktop, and handles providing the ScrollController to the Scrollbar for you.

final ScrollController controller = ScrollController();
ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
   return Text('Item $index');
 }
);

Some widgets in the framework are exempt from this automatic Scrollbar application. They are:

  • EditableText, when maxLines is 1.
  • ListWheelScrollView
  • PageView
  • NestedScrollView

Since these widgets manually override the inherited ScrollBehavior to remove Scrollbars, all of these widgets now have a scrollBehavior parameter so that one can be provided to use instead of the override.

This change did not cause any test failures, crashes, or error messages in the course of development, but it may result in two Scrollbars being rendered in your application if you are manually adding Scrollbars on desktop platforms.

If you are seeing this in your application, there are several ways to control and configure this feature.

  • Remove the manually applied Scrollbars in your application when running on desktop.

  • Extend ScrollBehavior, MaterialScrollBehavior, or CupertinoScrollBehavior to modify the default behavior.

    • With your own ScrollBehavior, you can apply it app-wide by setting MaterialApp.scrollBehavior or CupertinoApp.scrollBehavior.
    • Or, if you wish to only apply it to specific widgets, add a ScrollConfiguration above the widget in question with your custom ScrollBehavior.

Your scrollable widgets then inherits this and reflects this behavior.

  • Instead of creating your own ScrollBehavior, another option for changing the default behavior is to copy the existing ScrollBehavior, and toggle the desired feature.

    • Create a ScrollConfiguration in your widget tree, and provide a modified copy of the existing ScrollBehavior in the current context using copyWith.

Migration guide

Removing manual Scrollbars on desktop

Code before migration:

final ScrollController controller = ScrollController();
Scrollbar(
  controller: controller,
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
      return Text('Item $index');
    }
  )
);

Code after migration:

final ScrollController controller = ScrollController();
final Widget child = ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
    return Text('Item $index');
  }
);
// Only manually add a `Scrollbar` when not on desktop platforms.
// Or, see other migrations for changing `ScrollBehavior`.
switch (currentPlatform) {
  case TargetPlatform.linux:
  case TargetPlatform.macOS:
  case TargetPlatform.windows:
    return child;
  case TargetPlatform.android:
  case TargetPlatform.fuchsia:
  case TargetPlatform.iOS:
    return Scrollbar(
      controller: controller,
      child: child;
    );
}

Setting a custom ScrollBehavior for your application

Code before migration:

// MaterialApps previously had a private ScrollBehavior.
MaterialApp(
  // ...
);

Code after migration:

// MaterialApps previously had a private ScrollBehavior.
// This is available to extend now.
class MyCustomScrollBehavior extends MaterialScrollBehavior {
  // Override behavior methods like buildOverscrollIndicator and buildScrollbar
}

// ScrollBehavior can now be configured for an entire application.
MaterialApp(
  scrollBehavior: MyCustomScrollBehavior(),
  // ...
);

Setting a custom ScrollBehavior for a specific widget

Code before migration:

final ScrollController controller = ScrollController();
ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
   return Text('Item $index');
 }
);

Code after migration:

// MaterialApps previously had a private ScrollBehavior.
// This is available to extend now.
class MyCustomScrollBehavior extends MaterialScrollBehavior {
  // Override behavior methods like buildOverscrollIndicator and buildScrollbar
}

// ScrollBehavior can be set for a specific widget.
final ScrollController controller = ScrollController();
ScrollConfiguration(
  behavior: MyCustomScrollBehavior(),
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
     return Text('Item $index');
    }
  ),
);

Copy and modify existing ScrollBehavior

Code before migration:

final ScrollController controller = ScrollController();
ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
   return Text('Item $index');
 }
);

Code after migration:

// ScrollBehavior can be copied and adjusted.
final ScrollController controller = ScrollController();
ScrollConfiguration(
  behavior: ScrollConfiguration.of(context).copyWith(scrollbars: false),
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
     return Text('Item $index');
    }
  ),
);

Timeline

Landed in version: 2.2.0-10.0.pre
In stable release: TBD

References

API documentation:

Relevant issues:

Relevant PRs: