[multimode] Grid active cell turns blank

In [OWUM](https://github.com/mkromberg/owum), when a grid cell is selected, it turns blank. Not sure why this exists, we initially thought this is an issue with the multimode branch, but seems to be a pre-existing issue that we have missed.

One thing we can do to avoid this is to add some more tests for multimode.

Reproduce the blank active-cell behavior in OWUM using the multimode grid, then trace the grid selection rendering path. Compare the behavior with the existing multimode tests and add coverage for selecting a cell. Done means the selected cell remains visible and the relevant tests pass.

Written by the indexing model from the issue text.