Thanks for the suggestions. I use all them at various times, though I usually prefer the preview in a combined tab, because of the shorter line length. The basic issue is that using a wide screen I have more real estate available horizontally than vertically, so when I optimise for shorter lines, then I need to increase the editor area to avoid scroll bars.
It would be great if the editor could be dragged over to the right and docked there instead. The tabs as currently configured might turn into a multiline mess (if positioned at the top of a narrow column).
In any case, I put the idea out there in case it resonated with others. It is pretty common these days of wide screens for applications to have three columns. Personally, I prefer my email inbox and preview panes the same way (as columns, not rows).
In the meantime, there is always the ‘non-technical’ solution of rotating my widescreen to vertical orientation, then expanding the editor area as needed. 