Ghost lines appear after the table if vertically aligned cells are joined

[jademath]

To reproduce:

1. New document
2. Insert page-break
3. On the first page, create any tabular object with more than one row
4. Join any vertically lined cells
5. Ghost lines appear after the table

If the page rendering is changed to papyrus, then the ghost lines disappear.

[Utkal059]

Hi, I've been looking into this bug as part of my GSoC 2026 application. I reproduced it with a 2×2 table using row-span=2 on the top-left cell.
The root cause is that consumed sub-cells retain their border values after spanning is applied. When cell_box_rep::pre_display() runs, it draws borders for every cell including the consumed ones, creating ghost lines inside the merged region.
I've submitted a fix in a pull request that zeros the borders on consumed cells before boxes are constructed, and also guards each border side individually during rendering. Happy to discuss the approach with maintainers!