Part states (such as solar panel deployment) are not synced to other players. The parts appear to stay in their default (VAB/SPH) protovessel state, even when manually toggling them or using an action group key.

This is an issue because on one player's game a player's craft may be clear of a space station, but another player's game the player is smashing through the solar panel arrays.

This may also end up extending to syncing any part states such as infernal robotics joints or whatever else might be saved to the persistent file.

(I don't care if the action group key press itself is synced, but the end result for the affected parts should be.)

Solar panels not syncing even across many minutes in proximity as for 28a3ee9

I can partially confirm it (for solar panels and radiators from the interstellar mod).
However landing-leg-states seem to sync fine.
As for the action groups: I'm not experiencing any issues with the ActionGroupExtender.

I think we should attempt to do this one :)