Allow focusing (and/or resizing) only editor/preview

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


Sorry if I've misunderstood but isn't that what View > Layout button sequence > Editor / Viewer / Split View does?