As I need this and was tinkering with it I thought this is an opportunity to open an issue to ask questions before I open a PR in the coming days. My tinkering consisted of copying the entire TextBox and see if it's feasible with my knowledge to do this.

I managed

• typing multiple lines of text by removing the scaling from the text and changing the fillmode
• make the Caret aware of the Y-axis
• make the delete animation of characters aware of the Y-axis

I need to figure out

• vertical scrolling
• visually displaying a selection spanning multiple lines

Question time:

• Where should it be? Is it its own component sharing code with TextBox or is it just an option you can enable for the usual TextBox? I guess it'll be the former.
• What additional keys does it have to handle? One I know is the ability to add new lines and up/down keys for navigation.
• Does it need a scrollbar or any visual indicator that there is text outside of our vision? Something the normal TextBox could use as well maybe?
• What else super obvious does a TextBox with many lines need that I just overlooked?

Of course this won't ask/answer everything as code should be available for that but something fundamental (like the first question) may already be answered beforehand.