the validation code was assuming that:
- if the parent has overflow cells after a inserting divider cell
- the exact divider we are validating MUST be in those overflow cells
However, this is not necessarily the case. Imagine:
- First divider gets inserted at index `n`. It is too large to fit,
so it gets pushed to `parent.overflow_cells()`. Parent usable space
does not decrease.
- Second divider gets inserted at index `n+1`. It is smaller, so it
still fits in usable space.
Hence:
Provide information to the validation function about whether the inserted
cell overflowed, and use that to find the left pointer and assert accordingly.