LinesVisible Association

context LinesVisible inv:
	ViewPort : viewContents->size() =
	ViewPort : height.min(FileManager : document->size())

Displays Diagram

context LinesVisible inv:
	let topLineNumber = ScrollBar : handlePosition *
		FileManager : document->size()
	in ViewPort : viewContents = FileManager : document->
		subsequence(topLineNumber, topLineNumber + ViewPort: viewContents->size() - 1)