Inline decorations unexpectly become marks if user types at the same line

[xiaody]

https://stackblitz.com/edit/typescript-ngkt6v?file=index.ts,index.html,style.css

At setup, the "bdeco" text doesn't have any marks. But if I type after "zzz", the "bdeco" unexpectly gets a bold mark.

[marijnh]

Attached fix should prevent this issue.