#### Summary
Avoid a potential panic when rendering the active execution cell when
the allocated area has zero height.
#### Changes
- Guard rendering with `active_cell_area.height > 0` and presence of
`active_exec_cell`.
- Use `saturating_add(1)` for the Y offset to avoid overflow.
- Render via `active_exec_cell.as_ref().unwrap().render_ref(...)` after
the explicit `is_some` check.
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