It would be nice to have confirmation window asking user to confirm changing phase from investigators to mythos and vice versa like it is in FFG app.
It'd be nice to have it as an option. Some people may want to leave the existing state. But in general, I agree that it'd be a nice feature.
You can use the undo functionality
True, I like this feature in Valkyrie.
I still think it'd be good to have the feature.
Finally, I think it should be easy to implement; one event such as 'beforePhaseChange' (if it doesn't exist already) which checks if a certain flag is set and then, if needed, displays a simple 'yes'/'no' box in the centre of the screen.
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It'd be nice to have it as an option. Some people may want to leave the existing state. But in general, I agree that it'd be a nice feature.