fix: a table cell's metrics follow LibreOffice, not the browser

Two measured deviations, both worth ~1mm a row on a fixture: a cell's first block
got the space above that LibreOffice adds nowhere in a cell, and a repeating
header row rendered centred and bold because it is a <th> and the UA says so.
Pagination's page-top-drop detection now skips cell leaves, or the space that
is never there would read as a decoration to add back.

docs/architecture/tables.md

@@ -88,3 +88,15 @@ We deliberately keep TipTap's `resizable: false` (so its own columnResizing plug
   schematic text line (`previewTextCss`: thickness = bold, slant = italic, colour = the region's):
   without it an emphasis-only area such as first/last column or the total row is **invisible** in
   a tile, since the tiles hold no real text — only fills and borders would show.
+
+## Cell metrics measured against LibreOffice
+
+- **No space above a cell's first block.** LibreOffice adds none there (its
+  `AddParaTableSpacingAtStart`); Word does, so `editor.css` drops it for the `add` spacing
+  model only. Measured on the Math Guide: the table row grew 1mm per row without it. `pageBreaks.ts`
+  excludes cell leaves from its page-top-drop detection, or that missing padding would read
+  as a decoration to add back.
+- **`<th>` carries no UA defaults.** A repeating ODF header row (`table:table-header-rows`)
+  becomes a `tableHeader`, and the browser centres and bolds it — neither Word nor LibreOffice
+  does. `editor.css` resets both, so the cell follows its own style (the "header row" preset's
+  bold still comes from `.cell-header`).

src/lib/editor/extensions/pageBreaks.ts

@@ -807,8 +807,10 @@ export const PageBreaks = Extension.create({
                 const cs = getComputedStyle(child);
                 // The block's own space above, and how much of it the last pass took off
                 // at a page top (the decoration zeroes both padding and margin).
+                // A cell's first block never carries it (LibreOffice adds none there), so
+                // the missing padding is the document, not a decoration to add back.
                 const spaceAbove = parseFloat(cs.getPropertyValue('--space-before')) || 0;
-                const dropped = spaceAbove > 0.5
+                const dropped = spaceAbove > 0.5 && !inTableCell
                   && parseFloat(cs.paddingTop) < 0.5 && parseFloat(cs.marginTop) < 0.5
                   ? spaceAbove : 0;
                 leaves.push({

src/styles/editor.css

@@ -487,6 +487,12 @@
   padding-top: 0;
   margin-top: var(--space-before);
 }
+/* LibreOffice adds no space above the first block of a table cell (measured; its
+   AddParaTableSpacingAtStart), Word does — so it follows the document's spacing model. */
+.paper:not([data-spacing-model='max']) .tiptap :is(td, th) > :first-child {
+  padding-top: 0;
+  margin-top: 0;
+}
 
 /* The line box: the paragraph's spacing factor times its font's natural line height.
    Either half may come from the style rule or, nearer, from the block's own attrs
@@ -636,6 +642,10 @@
   border: 1px solid #000;
   padding: var(--cell-pad, 0 0.19cm);
   vertical-align: top;
+  /* A repeating header row is a <th>, and no word processor centres or bolds its text
+     for that reason — drop the two UA defaults so the cell follows its own style. */
+  text-align: inherit;
+  font-weight: inherit;
   /* Positioning context for the column-resize handle widget (a child div the
      resize plugin places at the cell's right edge). */
   position: relative;