The same page on every screen

The text is laid out on a page of one fixed size and the sheet scaled to
the screen, so a phone breaks it exactly where a desktop does — twenty-one
lines to the page on both. The type size is computed from the page and the
line count rather than chosen.

A PDF's full screen is the browser window, not the screen, and its button
sits beside the zoom. "Recent Notes" is stamped by every write, not only by
a rename. And a PDF that will not open says which of the four reasons it is.

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

e2e/editor.mjs

@@ -261,6 +261,76 @@ try {
     "and the gap stays blank — no placeholder printed at the foot of the page"
   );
 
+  /* ── twenty-one lines, and the same twenty-one everywhere ──
+   *
+   * Two claims. First, a page holds the number of lines a ruled page holds —
+   * it held eighteen on a phone, which is what started this. Second, and the
+   * one that matters: the phone breaks the text in exactly the same places the
+   * desktop does. It did not, because the text used to be set in screen pixels:
+   * a 340-pixel sheet fitted nothing like what a 760-pixel one did, so the same
+   * note had different lines on different pages depending on what you read it
+   * on. The text is now laid out on a page of a fixed size and the whole sheet
+   * scaled, so the only thing a narrow screen changes is how big it looks.
+   *
+   * Measured in page units — every length divided by the sheet's own scale —
+   * because screen pixels are precisely what must not agree. */
+  const pageUnits = () =>
+    p.evaluate(() => {
+      const scale = Number(document.querySelector("[data-text-page-scale]").dataset.textPageScale);
+      const pm = document.querySelector(".ProseMirror");
+      const cs = getComputedStyle(pm);
+      const sheet = document.querySelector("[data-page-sheet]").getBoundingClientRect();
+      const top = pm.getBoundingClientRect().top;
+      const blocks = [...pm.children]
+        .filter((el) => !el.classList.contains("ne-page-spacer"))
+        .slice(0, 40)
+        .map((el) => {
+          const r = el.getBoundingClientRect();
+          return { y: Math.round((r.top - top) / scale), h: Math.round(r.height / scale) };
+        });
+      return {
+        lines:
+          (sheet.height / scale - parseFloat(cs.paddingTop) - parseFloat(cs.paddingBottom)) /
+          parseFloat(cs.lineHeight),
+        blocks,
+      };
+    });
+
+  const onDesktop = await pageUnits();
+  check(
+    Math.floor(onDesktop.lines) === 21,
+    "a page holds twenty-one lines",
+    `${onDesktop.lines.toFixed(2)} fit`
+  );
+
+  const phone = { width: 390, height: 844 };
+  const desktop = p.viewportSize();
+  await p.setViewportSize(phone);
+  await p.waitForTimeout(2000);
+  /* A phone shows one pane at a time, so the note may have been left behind in
+     the list when the window narrowed. Open it again before measuring. */
+  if ((await p.locator("[data-text-page-scale]").count()) === 0) await reopenNote(p, "Langer Text");
+  await p.waitForSelector("[data-text-page-scale]", { timeout: 15000 });
+  await p.waitForTimeout(3000);
+  const onPhone = await pageUnits();
+  check(
+    Math.floor(onPhone.lines) === 21,
+    "on a phone as well — the same page, not a smaller one",
+    `${onPhone.lines.toFixed(2)} fit`
+  );
+  const moved = onDesktop.blocks
+    .map((b, i) => ({ i, b, o: onPhone.blocks[i] }))
+    .filter(({ b, o }) => !o || Math.abs(b.y - o.y) > 1 || Math.abs(b.h - o.h) > 1)
+    .map(({ i, b, o }) => `#${i}: ${b.y}/${b.h} vs ${o ? `${o.y}/${o.h}` : "missing"}`);
+  check(
+    onPhone.blocks.length === onDesktop.blocks.length && moved.length === 0,
+    "and every paragraph falls in the same place on both, to the page unit",
+    moved.slice(0, 4).join(" | ")
+  );
+  await p.setViewportSize(desktop);
+  await p.waitForTimeout(2000);
+  if ((await p.locator(".ProseMirror").count()) === 0) await reopenNote(p, "Langer Text");
+
   /* The note is still one document — pages are drawn, not stored. */
   const stored = (await api(p, "GET", "/notes")).body[0];
   const body = (await api(p, "GET", `/notes/${stored.id}`)).body.content;