feat(ui5-table-cell): merged property added

- New merged boolean property on ui5-table-cell
- Row borders refactored: border-top per cell instead of border-bottom per row
- First row top border / last row bottom border handled via :first-of-type / :last-of-type
- Selection highlight uses box-shadow instead of border-bottom to avoid layout shifts
- Table now uses inset-inline-end/start, the old left/right + :dir(rtl) removed
- When merged, cell's top border becomes transparent and content is hidden
- Merging is disabled on hover/focus, content reveals with an opacity transition
- Merging is also disabled when the row enters popin mode
- Implemented via CSS Space Toggle trick, no JS needed
- When a selection column is present, its border syncs with the first merged cell
- New TableCell website sample added for merged cells

- Partially fixes #7238
- CPOUIFTEAMB-2624

Author: aborjinik

packages/main/src/TableCell.ts

@@ -35,7 +35,8 @@ class TableCell extends TableCellBase {
 	 * Defines whether the cell is visually merged with the cell directly above it.
 	 *
 	 * This is useful when consecutive cells in a column have the same value and should visually appear as a single merged cell.
-	 * **Note:** This feature is disabled when cells are rendered as popin.
+	 * Although the cell is visually merged with the previous one, its content must still be provided for accessibility purposes.
+	 * **Note:** This feature is disabled when cells are rendered as popin, and should remain `false` for interactive cell content.
 	 *
 	 * @default false
 	 * @since 2.21.0

packages/website/docs/_components_pages/main/Table/TableCell.mdx

@@ -34,4 +34,6 @@ To ensure usability, merged cell styling is automatically disabled in these case
 - **Popin mode:** When the table is narrow and cells are rendered in the popin area, merged styling is removed so each row's content remains visible.
 - **Hover and focus:** When the user hovers over or focuses within the row, merged styling is temporarily removed, making the merged content visible.
 
+**Note:** Avoid using `merged` on cells that contain interactive content (e.g., buttons, or inputs).
+
 <MergedCells />