[oatmeal_coffee]

You can type directly into layouts in InDesign and see your line and page breaks in realtime. I think where the problem lies in InDesign is that it is, at its core, a typesetting and layout application and not a word processing application. There is a lot of work to do to get to the point of being able to write in your layout, and pages don't get added dynamically when adding new content as easily as they do in a typesetting application.

[munificent]

> pages don't get added dynamically when adding new content as easily as they do in a typesetting application.

InDesign will actually do this. I think it's called "Dynamic Flow" or something? It can automatically append new pages to fit the content.

In general, it's not very eager to do this because when you consider spreads (distinct left-hand and right-hand pages), inserting a new page will affect the layout of all subsequent pages. You could insert a new spread, but that may not be what the user wants.

In other words, if the user is laying out pages, actual pages matter. They aren't just an implementation detail derived from the length of the text.

[tvdijk]

You wouldn't necessarily be "writing in your layout". You could do something where you have a separate preview pane that updates in real time. Similar to what Apple showed earlier this year with their Swift playground IDE, only for typesetting.