Cross-posting for context:

In T332124#8827194, @egardner wrote:

I still have some questions about how the subheading would appear and disappear in this design:

• If the heading is fixed with a subheading below it, I assume that we don't want it to slide out of the top of the dialog frame as the user scrolls – the two lines of text would collide and it would look jarring. So would the subheading just disappear immediately (maybe with a quick fade) on scroll?
• I assume we want to keep the text inside the main dialog content area from jumping around when the subheading disappears and later re-appears. This means that we probably will need to use fixed positioning for the header and footer sections of the dialog – this ensures that these elements won't impact the layout of the main content area as they change. But using fixed positioning for the header and footer might cause other issues, and could complicate the ability of users to provide custom header/footer content and have things just work as expected.

My gut feeling is that this proposed design is going to require a lot of effort to implement and may produce side-effects that force us to re-think other aspects of the Dialog component. My recommendation would be to consider an alternative (simpler) approach that does not necessitate multiple elements appearing and disappearing on scroll. Maybe we should just re-introduce the optional dividers and avoid other changes for now?