Alright, I debugged it over the weekend and my assumptions were partly right.
Bug: On nix platform, Render function was called inconsistently, so io.DeltaTime was <= 0ms
Cause: Since VTK doesn’t call Render in every iteration of the event loop, I had to create a one-shot timer for 0ms so that the event queue will always have a TimerEvent that will call my render function. This is to make sure the overlay is drawn on every frame. Apparently, the x11 Render window interactor doesn’t behave well with 0ms timers, as a matter of fact, it doesn’t even fire the timer.
I’ll have to delegate vtkDearImGuiInjector to call ProcessEvents effectively taking over the event loop. This could be achieved if interactor framework exposed HandleEventLoop via vtkBooleanMacro. I want to do this without deriving vtkRenderWindowInteractor for simplicity at the injection site. Since it’s default value is false, its okay for now!