Compare map widget sometimes fails

There's some weird timing bug with `lonboard` / DeckGL during the layer construction when two maps are initted at the same time that causes the static output of the Map Compare widget to "sometimes" fail - it's a bit random and hard to reproduce, but creating this issue to dump notes with some further digging.

Start with the Map Compare widget and the layer-construction path involving lonboard and DeckGL. Try initializing two maps at the same time and observe when the static output fails. Done means the Map Compare static output is reliable when two maps are initialized concurrently.

Written by the indexing model from the issue text.