Magento's OPC splits the interaction into two components:
- Accordion of checkout steps
- Progress sidebar
As this structure is heavily tied to JS behavior, changing it requires accepting a significant maintenance burden and introduces conflicts with third-party code.
Redesigning OPC from a mobile-first perspective presents a better solution: woven checkout. As each OPC step is completed its form is collapsed and replaced by the progress summary for that step. This is ideal for multiple reasons:
- Natural Hierarchy — the editing interface erodes as the progress interface expands. It's a logical tide exchanging input for feedback.
- Proximity — there's no interface gap between input, reviewing, and editing. For each step, all the interaction modes occupy the same physical space. This connectedness makes the interface more intuitive and consistent.
- Device Friendly — Scales up from mobile to desktop without drastic layout transformations.
In Magento 1.8 / 1.13 the OPC progress sidebar was rewritten to pull each progress state from a separate AJAX request. It should be possible to implement woven checkout without significantly re-architecting OPC.
The pattern should be adopted in the core because third-parties building checkout extensions need to have a standard to build to.