If you ever want to find out how to do something with Scintilla, your first stop should always be the core Scintilla Documentation. It is comprehensive, and usually kept fully up to date.
The correct way to do what you want is to use the SCI_GETFIRSTVISIBLELINE message to get the first line, and then use the SCI_LINESONSCREEN message to calculate the last line.
There are probably Scintilla.NET wrapper methods for those messages. But the Scintilla.NET documentation seems very poor, and doesn't provide a complete description of its API - although I suppose you could always use the SendMessageDirect
method (which is documented) to send the messages directly if you can't guess what the wrapper method is called.