TOAD & Fischland on Mac OS X
I finally came up with a strategy to extend TTextArea with HTML support: Keep it simple and start with an editor which just edits a single line and displays the internal and rendered representations at once. About two years ago I tried to do the same directly with TTextArea and ended up in debugging hell.
This time it looks like I'm going have something I can easily integrate into TTextArea, replace THTMLView with, use it in the vector graphics editor for text fields and speech bubbles, a collaborative real-time editor, a screenwriting app, ...
And additional data structures might only be required for the paragraphs displayed on the screen.