Editor prepaint previously used the full parent content mask height when
deciding which rows to lay out, while only accounting for top clipping.
Embedded, content-sized editors in agent tool cards could therefore ask
the display map to highlight rows below the editor's visible
intersection with the list viewport. Compute the vertical intersection
between the editor bounds and the content mask instead, so highlighted
chunks and custom highlight endpoints are built only for rows that can
actually be painted.

Release Notes:

- N/A or Added/Fixed/Improved ...