Tunneled dialogs were introduced in LOOL 6.1 .
A couple of examples: Character, Paragraph, Spelling, Search and Replace, Format Cell etc.
AutoFilter dropdowns are also tunneled.
A few resources.
Pranav Kant's blog, and talk at LibOCon 2017:
- https://conference.libreoffice.org/2017/the-program-2/oct-12th-thursday/index.html - Tunneling dialogs to LibreOffice Online
Kendy's talk at FOSDEM 2018: