This is by design. ImGUI stops forwarding mouse events to VTK when the mouse is inside imgui window. But I think this is not expected of an overlay! Thanks for mentioning that! I will add a checkbox to grab mouse input.
About the imgui assertion error, I’ll debug it on a linux box and see what I can do… maybe it has to do with the implementation of chrono::system_clock on nix vs windows c++ libraries.