Centre the writing column again

Pinning it to the left edge put the start of every line in the same
place, which was the point, but on a wide window it left the entire
right half of the pane as one dead block. Centred, the same leftover
space reads as two margins instead.

The page surface that tried to give that block a meaning is reverted in
the commit before this one: it made the whole sheet sit hard against the
left edge, which was worse than the problem.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

e2e/shot.mjs

@@ -0,0 +1,22 @@
+/* Throwaway: one wide screenshot of the writing surface, to look at it. */
+import { OUT, launch, withFreshUser, createNotebook, createNote, typeInEditor } from "./lib.mjs";
+
+const browser = await launch();
+const session = await withFreshUser(browser, "shot");
+const p = session.page;
+await p.setViewportSize({ width: 1680, height: 900 });
+await createNotebook(p, "Look");
+await createNote(p);
+await typeInEditor(p, "askjdnasjkdnkjasndjknkjaskjdnasjkdskajndjkanHelkl HALLLO");
+await p.keyboard.press("Enter");
+await p.keyboard.type("sdfsdfsdfsdfasddddd");
+await p.waitForTimeout(1500);
+await p.screenshot({ path: `${OUT}/look.png` });
+const box = await p.evaluate(() => {
+  const pm = document.querySelector(".ProseMirror").getBoundingClientRect();
+  const pane = document.querySelector(".note-editor").parentElement.getBoundingClientRect();
+  return { pm: { x: Math.round(pm.x), w: Math.round(pm.width) }, pane: { x: Math.round(pane.x), w: Math.round(pane.width) } };
+});
+console.log(JSON.stringify(box));
+await session.cleanup();
+await browser.close();

frontend/src/editor/editor.css

@@ -38,10 +38,11 @@
      point where the eye reliably finds the start of the next line. Capped here
      rather than on the container so it holds on a page as well as off one. */
   max-width: 527px;
-  /* Left, not centred. A column that floats in the middle moves as the window
-     is resized, so the start of every line lands somewhere new; against the
-     margin it is always in the same place. */
-  margin-left: 0;
+  /* Centred. Pinning the column to the left edge put every line in the same
+     place, but on a wide window it left the whole right half of the pane as one
+     dead block — which is worse to look at than a column that moves. Centred,
+     the same leftover space reads as two margins. */
+  margin-left: auto;
   margin-right: auto;
   outline: none;
   color: var(--ne-text);