fix: pagination measures a text box's own spacing

The atomic leaf a text box becomes carried neither its space above nor its space
below, so the fit test ignored both and a page top never dropped the space above
the way it does for a paragraph. The box's space rides --space-top, which the
page-top decoration zeroes — the rotor centres itself in the padding box and has
to follow it.

src/lib/editor/extensions/pageBreaks.ts

@@ -751,13 +751,22 @@ export const PageBreaks = Extension.create({
               // A text box paginates atomically: a line-split spacer can't render
               // sanely inside a rotated/floated node view.
               if (child.classList.contains('textbox-node')) {
+                // Its anchor paragraph's spacing rides the box (textBox.ts): space above
+                // as padding, which a page top drops, space below as the margin.
+                const boxCs = getComputedStyle(child);
+                const spaceAbove = parseFloat(boxCs.getPropertyValue('--space-before')) || 0;
+                const dropped = spaceAbove > 0.5 && !inTableCell && parseFloat(boxCs.paddingTop) < 0.5
+                  ? spaceAbove : 0;
                 leaves.push({
                   el: child,
                   kind: 'atomic',
                   naturalTop: naturalTopOf(child),
-                  naturalHeight: child.offsetHeight,
+                  naturalHeight: child.offsetHeight + dropped,
+                  spaceAfter: inTableCell ? 0 : parseFloat(boxCs.marginBottom) || 0,
+                  spaceAbove,
                   inTableCell,
                 });
+                cumulativeDropped += dropped;
                 continue;
               }
               // A multi-column section fragment: also atomic for spacer purposes (a
@@ -1316,7 +1325,8 @@ export const PageBreaks = Extension.create({
               }));
             }
             for (const b of pageTopBlocks) {
-              decoArray.push(Decoration.node(b.from, b.to, { style: 'padding-top:0;margin-top:0' }));
+              // --space-top: a text box draws its own space above from it (textBox.ts).
+              decoArray.push(Decoration.node(b.from, b.to, { style: 'padding-top:0;margin-top:0;--space-top:0px' }));
             }
 
             decorations = decoArray.length > 0

src/lib/editor/extensions/textBox.ts

@@ -388,11 +388,11 @@ class TextBoxView {
     // frame distance already is.
     const before = this.node.attrs.spaceBefore as number | null;
     const after = this.node.attrs.spaceAfter as number | null;
-    // On the rotor too: --space-before is a registered non-inheriting property, and the
-    // rotor centres itself in the padding box, so it needs the value to correct for it.
-    for (const el of [d, this.rotor]) {
-      if (before != null) el.style.setProperty('--space-before', `${before}pt`);
-      else el.style.removeProperty('--space-before');
+    // --space-top is the same value the rotor can inherit (--space-before is registered
+    // non-inheriting), and it is what pageBreaks.ts zeroes at a page top.
+    for (const prop of ['--space-before', '--space-top']) {
+      if (before != null) d.style.setProperty(prop, `${before}pt`);
+      else d.style.removeProperty(prop);
     }
     if (after != null) d.style.marginBottom = `${after}pt`;
     // Set against the middle or the far end of the column, unless the file placed the

src/styles/editor.css

@@ -387,8 +387,8 @@
   position: absolute;
   left: 50%;
   /* Centred in the padding box, so half the space above has to come back to keep the
-     frame itself below it. */
-  top: calc(50% + var(--space-before) / 2);
+     frame itself below it — the *applied* space, which a page top drops. */
+  top: calc(50% + var(--space-top, 0px) / 2);
   box-sizing: border-box;
   transform: translate(-50%, -50%);
 }