I am working on Google Map Markers in Flutter.
On the click of each Marker, I want to show a Custom Info Window which can include a button, image etc. But in Flutte
I stumbled across the same problem just today, I couldn't get a multiline string to show properly in TextInfoWindow. I ended up circumventing the problem by implementing a modal bottom sheet (https://docs.flutter.io/flutter/material/showModalBottomSheet.html) that shows when you click on a marker, which in my case worked out quite nicely.
I can also imagine many use cases where you'd want to fully customize the marker's info window, but reading this issue on GitHub (https://github.com/flutter/flutter/issues/23938) it looks like it's currently not possible, because the InfoWindow is not a Flutter widget.