Currently, we have this mockup for horizontal layouts.

There are some things left unclear:

• what about fields that do not stretch, like checkboxes?
• what to do when there are too many fields? Some options to consider from easiest to more expensive:
  • write in the documentation that that's up to the user to limit it reasonably, they can rely on widening the dialog but then users with limited screens won't reach the fields 
  • use some container, where overflowing fields will be horizontally scrollable
    • this could be visually unappealing
• what to do about nesting
  • one of the fields being composite
    • and set the layout to be non-horizontal/horizontal
  • or worse a multi — these examples clearly show that the user would be stretching the functionality to test it limits rather than looking for a reasonable setup.
• multi fields have buttons to the right of the fields, stealing some space. If a multi's inner field is designed to be horizontally laid out, the layout needs to account for the presence of those buttons. This was already filed as a bug here: MGNLUI-6399 The design needs to account for possibly narrowed spaces like being a nested multifields's entry or generally, being in a dialog