Darren,
1.59.1 is out, and everything you drew arrows at is in it. Before the list, though, the part I owe you.
Your arrows were pointing at something I had not fixed, and I had told you twice that I had.
The first time, I changed the Side by Side margins to a value that shrinks on a narrow screen — and never worked out what it came to on a real one. On your Air it came to 22 points: the number it already was. The second time I widened the reading column at Full width and told you the borders were dealt with again — but that change went to the single editor and never reached Side by Side, which keeps its own margins entirely. So the space either side of each column was the same throughout, and you asked three times about a gap that had never once moved.
It is a plain twelve points now, on every display, no formula. Two columns pay for their margins twice, which is precisely why you feel it and I did not: that is around 88 points of width back on your screen.
And the header, as you drew it. The titles, the ✕ and the focus line sit in a row about a third as tall, with the title a size smaller. That row sits between the toolbar and your first line of writing, twice over, so its height was never a detail.
The two Full Screen items are gone too — or rather ours is, and the system’s stands alone. macOS puts its own full-screen item into any menu called View, and MojoPad was declaring one as well, under the same label, which is why they read as an exact duplicate rather than merely a spare. The system’s also renames itself to Exit Full Screen once you are in it, which ours never did. One thing it costs: ⌃⌘F is no longer the shortcut — the system’s is fn F. I would rather tell you that than have you find it.
Not picky at all. Four times now you have asked for the same thing in different clothes — the thick bar, the borders, the file toolbar, and now this — and every one of them was a small screen giving too much of itself to something other than writing. The mock-up was the most useful of the four, because I could see what you meant instead of working it out.
It should offer itself to you. If you find anything else while you are trying it properly, that is good timing rather than bad.
Mark