Expand/Collapse states for groups are reset if Gui.over is used in user's update

[shamansir]

So if there's animation tick among the messages, the UI becomes useless :(

[shamansir]

Probably we may do:

collectExpands : Builder msg -> Dict Path ExpandState

applyExpands : Dict Path ExpandState -> Builder msg -> Builder msg

inside Gui.over.

But also recommend not to use over too often — try to minimise its usage to only the cases when model affects the GUI structure.