diff --git a/admin-lehrer/components/grid-editor/GridTable.tsx b/admin-lehrer/components/grid-editor/GridTable.tsx
index e76c979..a5dfe82 100644
--- a/admin-lehrer/components/grid-editor/GridTable.tsx
+++ b/admin-lehrer/components/grid-editor/GridTable.tsx
@@ -52,13 +52,18 @@ export function GridTable({
   // ----------------------------------------------------------------
   // Compute column widths from OCR measurements
   // ----------------------------------------------------------------
-  const zoneWidthPx = zone.bbox_px.w || layoutMetrics?.page_width_px || 1
-  const scale = containerWidth > 0 ? (containerWidth - ROW_NUM_WIDTH) / zoneWidthPx : 1
-
-  // Column widths in original pixels, then scaled to container
+  // Use the actual total column span as reference width — NOT zone.bbox_px.w.
+  // When union columns are applied across content zones, column boundaries
+  // can extend beyond the zone's bbox, causing overflow if we scale by
+  // the smaller zone width.
   const [colWidthOverrides, setColWidthOverrides] = useState<number[] | null>(null)
 
   const columnWidthsPx = zone.columns.map((col) => col.x_max_px - col.x_min_px)
+  const totalColWidthPx = columnWidthsPx.reduce((sum, w) => sum + w, 0)
+  const zoneWidthPx = totalColWidthPx > 0
+    ? totalColWidthPx
+    : (zone.bbox_px.w || layoutMetrics?.page_width_px || 1)
+  const scale = containerWidth > 0 ? (containerWidth - ROW_NUM_WIDTH) / zoneWidthPx : 1
 
   const effectiveColWidths = (colWidthOverrides ?? columnWidthsPx).map(
     (w) => Math.max(MIN_COL_WIDTH, w * scale),