Write on paper: a sheet per page, with a gap between them

The writing surface was a column of text in an empty pane. A sheet was
drawn behind it, but only once you started drawing — so writing looked
like text floating in the middle of nothing, and the space either side of
the column read as a void rather than as a margin.

Now every page is a sheet of its own, centred, with a gap after it: the
same way the PDF reader draws its pages, because a page in a note and a
page in a document should read as the same kind of thing.

The gap is the part that takes work. The browser knows nothing about the
sheets and lays the text out as one flow, putting a line straight across
the gap and cutting it in half. So editor/pagination.js measures the flow
and pushes any block that would straddle a gap onto the next page, with a
spacer of exactly the right height in front of it. A paragraph moves down
entire, which is what a page break means on paper. A block taller than a
page is left to run over, since splitting one would mean laying out its
insides ourselves.

The spacers are decorations, not content: they never enter the document,
never reach the server and never appear in an export. Two checks hold
that down — one asks whether any block sits in a gap, the other reads the
stored body back and looks for them there.

Two things that are easy to get wrong. The spacer's height is not the
distance the text moves: margins that used to collapse into each other no
longer do with something between them, so the layout gains that much for
free and the spacer is made exactly that much shorter. And heights change
with no transaction to announce it — a webfont landing, an image
arriving, the window resizing — which a ResizeObserver on the editor
element catches.

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

e2e/probe.mjs

@@ -0,0 +1,78 @@
+/* Throwaway: four pages, and what that stray placeholder is. */
+import { launch, withFreshUser, createNotebook, createNote, OUT } from "./lib.mjs";
+
+const browser = await launch();
+const s = await withFreshUser(browser, "probe");
+const p = s.page;
+p.on("pageerror", (e) => console.log("PAGEERROR", String(e)));
+
+await p.setViewportSize({ width: 1680, height: 950 });
+await createNotebook(p, "Papier");
+await createNote(p);
+await p.waitForTimeout(1500);
+await p.locator(".ProseMirror").click();
+// Paste rather than type: typing this much loses keystrokes and says nothing
+// about the layout, which is what is being looked at here.
+await p.evaluate(() => {
+  const text = Array.from({ length: 160 }, (_, i) => `Absatz ${i + 1}: Dichtungen altern durch Wärme und Zeit.`).join("\n");
+  const dt = new DataTransfer();
+  dt.setData("text/plain", text);
+  document.querySelector(".ProseMirror").dispatchEvent(
+    new ClipboardEvent("paste", { clipboardData: dt, bubbles: true, cancelable: true })
+  );
+});
+await p.waitForTimeout(4000);
+
+console.log(
+  JSON.stringify(
+    await p.evaluate(() => {
+      const pm = document.querySelector(".ProseMirror");
+      const pmTop = pm.getBoundingClientRect().top;
+      const sheets = [...document.querySelectorAll("[data-page-sheet]")];
+      const gaps = sheets.slice(0, -1).map((el, i) => ({
+        from: el.getBoundingClientRect().bottom - pmTop,
+        to: sheets[i + 1].getBoundingClientRect().top - pmTop,
+      }));
+      const inGap = [];
+      for (const el of pm.children) {
+        if (el.classList.contains("ne-page-spacer")) continue;
+        const r = el.getBoundingClientRect();
+        if (r.height === 0) continue;
+        const top = r.top - pmTop,
+          bottom = r.bottom - pmTop;
+        for (const g of gaps) {
+          if (bottom > g.from + 0.5 && top < g.to - 0.5) inGap.push((el.textContent || "").slice(0, 24));
+        }
+      }
+      const placeholder = [...pm.querySelectorAll("[data-placeholder]")].map((el) => ({
+        text: (el.textContent || "").slice(0, 20),
+        first: el === pm.firstElementChild,
+        top: Math.round(el.getBoundingClientRect().top - pmTop),
+      }));
+      return {
+        blocks: [...pm.children].filter((c) => !c.classList.contains("ne-page-spacer")).length,
+        height: pm.scrollHeight,
+        sheets: sheets.length,
+        spacers: [...document.querySelectorAll(".ne-page-spacer")].map((el) => el.offsetHeight),
+        inGap,
+        placeholder,
+        position: document.querySelector("[data-note-position]")?.textContent,
+        margins: (() => {
+          const sheet = sheets[0].getBoundingClientRect();
+          const col = pm.getBoundingClientRect();
+          return {
+            sheetW: Math.round(sheet.width),
+            colW: Math.round(col.width),
+            left: Math.round(col.left - sheet.left),
+            right: Math.round(sheet.right - col.right),
+          };
+        })(),
+      };
+    }),
+    null,
+    1
+  )
+);
+await p.screenshot({ path: `${OUT}/probe.png` });
+await s.cleanup();
+await browser.close();