This guy has solved his custom Quill toolbar add-on:

React Quill add line-height to custom toolbar

https://medium.com/@genyadstph/react-quill-add-line-height-to-custom-toolbar-91a8717394c8

I also found this:

Replace 'p' tag with 'div' by adding the the following lines. It will solve your mail spacing issue.

 var Block = Quill.import('blots/block');

           Block.tagName = 'div';

           Quill.register(Block);

Line 1

Line 2

Line 3

Interesting: Selecting all text right here in this discussion block and changing the format to header 6 removes the spacing.