When a latex document grows large enough, it becomes practically impossible to edit with Scintilla-based editors (e.g. notepad++). The issue may be avoided by hitting return on every line which is not a solution for many people.
Steps to reproduce:
1. open the file attached and ensure TEX-highliting is ON.
2. ensure line wrapping is ON.
3. type quickly something like "asdfasdfasdf" in the beginning or in the middle of the text.