I want to add characters to the display of my paragraphs without modifying the underlying text storage. To be concrete, let's say that I want to prefix each paragraph with "Whereas, ". Further, when I drag out a text selection on a paragraph that's displayed this way, I do not want "Whereas, " to receive a selection highlight.
Using TextKit 2, it's pretty easy to accomplish the first part using a NSTextContentStorageDelegate whose method textContentStorage(_: NSTextContentStorage, textParagraphWith: NSRange) -> NSTextParagraph? adds my prefix to each paragraph. I modified the WWDC21 demo program to do it. I appreciate how I can avoid glyph munging by using TextKit 2 instead of TextKit 1, it will save a lot of trouble.
If I drag a selection across the prefix, then it receives the selection highlight. What's more, when I drag a selection across the end of the paragraph, the last eight characters (and, I presume, the invisible newline) do not receive the selection highlight. So adding the prefix has skewed the selection indices.
It seems to me that the TextKit 2 designers may intend for me to use a NSTextLocation subclass consisting of a "storage" index and a "display" sub-index. On the storage indices there are the characters of the text storage, and at (storage index, display sub-index)-pairs are the characters of my prefix. While the user drags out a selection, I should subtract from the NSTextSelection any NSTextRanges belonging to non-storage text such as my prefix.
Am I on the right track, so far?