Is it possible to make the menu bar that contains “Open in PDF editor …”/“Open in default app..”, “Open in Split View”, “Book Notes”, “Reload” much more compact? It takes up a lot of screen real estate on the my MacBook Air.
Darren,
All four of these are in 1.59.0, and I owe you a correction on the first one.
The borders. When you said the writing space was unchanged, you were right, and my earlier fix is why. I had changed the Side by Side margins from a fixed 22 points to a value that shrinks on a narrow screen — and then never checked what it came to on a real one. On a MacBook Air it works out to 22 points: exactly the number it already was. It only does anything on a window narrower than any Mac you would actually work in. So you thanked me for a fix that changed nothing on your screen, and then reported the same problem, and both of those were correct.
The real cause was somewhere else entirely and affects every window, not just Side by Side. Each editor kept a floor of 44 points of margin on each side regardless of the width you chose. At Small, Medium and Large you never see it — the reading column is narrower than that anyway. But at Full the calculation comes to nothing and the floor is the margin, so choosing full width got you 88 points of the window you had just asked for. That floor is 20 points now, and only at Full; every other width is untouched.
Dragging. The divider between the two columns drags now, and remembers where you left it. Double-click it to go back to half and half. The read-only pane has had that for ages and Side by Side simply never did, which is not a distinction anyone could have guessed.
Side by Side versus Split View — you were not missing anything. They were one feature introducing itself twice. Underneath, opening a split view with the right setting on became Side by Side, so the same choice did different things depending on a preference you may never have set. There is one command now, Open Beside This Page…, and what you get depends on the page rather than on which menu item you found: a page you can write in opens ready to write in, and a PDF, a book or a locked page opens as a reading pane. You can turn a reading pane into a second editor at any time with the 
in its header — which now only appears where it can actually do something, instead of sitting on PDFs and refusing.
And your Scrivener point: a hairline under a column’s name now shows which one has the cursor. You were right that with two editable panes it needs saying.
The bar above PDFs and books is about a third shorter — same buttons, same order, it just stops announcing itself. It is above the document the whole time you are reading, so its size was not a small thing.
No apology for the Scrivener references. Knowing exactly what good feels like and being able to point at it is the most useful thing a reader can bring.
Mark