[RFC] Deprecate "down" responsive behavior

[kettanaito]

What:

I suggest to deprecate the "down" behavior of responsive props.

Why:

I have two concerns with the "down" behavior:

1. Desktop-first is not a good practice

Mobile-first has been a strong encouragement and industry standard for years now. Modern technologies also support this approach and teach developers to work with it (i.e. Bootstrap). It would be good to guide atomic-layout users to develop mobile-friendly layouts as the primary way of responsive props assignment. In this case less customization would be beneficial to the end implementation. This doesn't mean deprecation of the "only" behavior.

2. "Down" behavior is a technical cornerstone

• See Prop with explicit "down" behavior overrides implicit "up" behavior of preceding prop #119

Making this behavior work per-spec would imply more complex logic between mobile-first properties (which are encouraged by default). I would like to prevent support of features that demand serious refactoring without significant value to the library and end usage (such being Internet Explorer, for example).

How:

• Deprecate the behavior before next major release. (Adds deprecation warning on "down" behavior usage #168)
• Remove the behavior in the next major release. Update release notes and documentation to reflect the change.

[kettanaito]

After numerous attempts to tackle this, I think the "down" behavior is here to stay. It's a powerful way to provide a cancellation behavior, and its alternative using "up" responsive props would be too verbose and unnecessary.

There are, however, changes to be made in the expectations toward the "down" responsive behavior:

1. The "down" behavior should be considered as an explicit override, not the recommended way to evolve props permutations.
2. The "down" behavioral responsive props are sensitive to declaration order, meaning that would override the preceding "up" responsive props, if met. In general, think of responsive props declaration as of CSS properties declarations (those are also sensitive to the declaration order).

This feature needs meaningful examples of usage to close this issue.

[kettanaito]

Conclusion

I have not found sufficient use cases and evidence that would support my idea of removing the "down" responsive prop behavior. So far it's quite useful in a lot of cases and I would prefer it to stay that way. Although its application is not always evident (#119), it can be systematized and understood through documentation.

Closing is for now, unless stronger evidence are found.