compose: Compose-box border is subtle in dark mode

[lakshya1goel]

Updated the top border and added shadow effect to the top of the compose box to ensure it is subtle in dark mode.

For reference, see the Figma design:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5908-64038&t=alSmAmdRXFDwT4IT-1

Fixes: #1207

Before	After

[chrisbobbe]

Thanks! I see this is a draft, but I gave it a quick skim and I noticed a different direction I'd like to take. 🙂

Since the shadow is only meant to be applied in dark theme, I'd like to avoid asking Flutter to do any computations for it when in light mode. For that, let's add a ComposeBoxTheme class in lib/widgets/compose_box.dart, with just one field boxShadow that's null in light theme and non-null in dark theme. ContentTheme and MessageListTheme are examples of the pattern I have in mind. So, something like this:

Proposal

/// Compose-box styles that differ between light and dark theme.
///
/// These styles will animate on theme changes (with help from [lerp]).
class ComposeBoxTheme extends ThemeExtension<ComposeBoxTheme> {
  factory ComposeBoxTheme.light(BuildContext context) {
    return ComposeBoxTheme._(
      boxShadow: null,
    );
  }

  factory ComposeBoxTheme.dark(BuildContext context) {
    return ComposeBoxTheme._(
      boxShadow: [BoxShadow(
        color: DesignVariables.dark.bgTopBar,
        offset: const Offset(0, -4),
        blurRadius: 16,
        spreadRadius: 0,
      )],
    );
  }

  ComposeBoxTheme._({
    required this.boxShadow,
  });

  /// The [ComposeBoxTheme] from the context's active theme.
  ///
  /// The [ThemeData] must include [ComposeBoxTheme] in [ThemeData.extensions].
  static ComposeBoxTheme of(BuildContext context) {
    final theme = Theme.of(context);
    final extension = theme.extension<ComposeBoxTheme>();
    assert(extension != null);
    return extension!;
  }

  final List<BoxShadow>? boxShadow;

  @override
  ComposeBoxTheme copyWith({
    List<BoxShadow>? boxShadow,
  }) {
    return ComposeBoxTheme._(
      boxShadow: boxShadow ?? this.boxShadow,
    );
  }

  @override
  ComposeBoxTheme lerp(ComposeBoxTheme other, double t) {
    if (identical(this, other)) {
      return this;
    }
    return ComposeBoxTheme._(
      boxShadow: BoxShadow.lerpList(boxShadow, other.boxShadow, t)!,
    );
  }
}

Then the caller can say boxShadow: ComposeBoxTheme.of(context).boxShadow.

Note the use of DesignVariables.dark.bgTopBar instead of adding a new design variable. The Figma doesn't add any new design variables. Also, it's DesignVariables.dark.bgTopBar instead of DesignVariables.dark().bgTopBar because of a small optimization, #1236; I suggest rebasing your work atop that PR to get that optimization.

[lakshya1goel]

Thanks for the review, I am working on it and will update it as required ASAP.
Thanks!