Temporary input is shown with wrong font, size and position. (IME's default font?)
I assume this is because you are using Windows' built-in text display engine but Affinity's for this part unlike Mac version?
It's little inconvenient and sometime it will be a problem to guess the final output.
I know implementing custom text display logic like Mac version is tough and problematic.
But I would like you to implement it like Mac version at some point.