Enable BoxDecoration for DefaultTextBlockStyle of header Attribute

[satotoshitaka11]

Is there an existing issue for this?

• I have searched the existing issues

Use case

In the current implementation of Flutter Quill, it is not possible to apply a BoxDecoration to the header attribute's DefaultTextBlockStyle. This limitation prevents users from customizing the appearance of headers, such as adding borders, background colors, or other decorations.

Adding a BoxDecoration to the header block, as shown in the attached screenshots, would improve the flexibility of customization.

Current	Desired

Proposal

Add support for BoxDecoration in the DefaultTextBlockStyle of the header attribute.

Refer to the images in the Use case section for a visual comparison.

[satotoshitaka11]

Since I am unable to push the changes directly, I have attached the modified files to this issue for your review.

Please kindly check the attached files for the necessary changes.

flutter_quill_header_style_update.zip