The responsive layout builder is currently working, for a lot of use cases, but falls down in a few places. Fixing this issue means:
1. I can move the Sidebar A region from the left side of the Body region to the right side of the Body region.
2. Moving regions around on rows will accurately reflect visual order in the regionList override data passed back to Drupal, so this layout information can be saved and converted to correct CSS.
3. I will be warned if my attempts to move a region will cause DOM element reordering.
The work on this feature will take place in the Layout module issue queue. This issue will catalog sub-issues related to it.