Problem

When using a Scaffold widget, any parent DefaultTextStyle will subsequently be ignored:

DefaultTextStyle.merge(
  style: TextStyle(
    letterSpacing: -0.5,
  ),
  child: Scaffold(
    body: Center(
      // The default text style set above is ignored here.
      child: Text('Foo'),
    ),
  ),
)

Use case

This entirely prevents setting a global default text style (e.g. to adjust the letter spacing for all text widgets in an app) when using Scaffold at all (which is usually used on every page in a Flutter app).

Cause

The reason this happens is because every Scaffold inserts a Material widget and this in turn overrides the DefaultTextStyle with TextTheme.bodyMedium:

This is also not documented. Neither the Scaffold widget mentions this behavior, nor the Material widget itself. Material.textStyle only states the following (and not that omitting the parameter will override DefaultTextStyle):

The typographical style to use for text within this material.

The only mention of this in the docs I could find was in TextTheme.bodyMedium:

The default text style for Material.

This, however, does not help the problem at all as there is no way to prevent the Material widget from overriding the DefaultTextStyle when using a Scaffold. There is no parameter for passing this. And even if there was one, we would have to manually pass our default text style to every Scaffold or Material usage within the app.
To me, this seems to be a fundamental issue.

Proposal

I propose the following:

1. The documentation should be fixed.

And any (or a combination) of the following should be implemented:

1. A flag on the app's Theme/TextTheme passed to MaterialApp that disables defaulting to bodyMedium if the app does not use bodyMedium.
2. A flag on the Scaffold widget to respect the DefaultTextStyle.
3. A way to prevent Material from overriding the default text style.