Either or both a and b are fine for me.
a) have 2 buttons or 1, that toggles between the three views (full edit, full preview, both); to focus either preview or editor (making one of them take up the space of both)
b) allow dragging/adjusting the size between editor and preview
#gsoc-2020
Sorry if I've misunderstood but isn't that what View > Layout button sequence > Editor / Viewer / Split View does?
