Tunneled dialogs were introduced in LOOL 6.1 [1]. A couple of examples: Character, Paragraph, Spelling, Search and Replace, Format Cell etc. AutoFilter dropdowns are also tunneled. [1] https://wiki.documentfoundation.org/ReleaseNotes/6.1#Online
A few resources. Pranav Kant's blog, and talk at LibOCon 2017: - http://pranavk.me/open-source/dialog-tunneling/ - http://pranavk.me/open-source/dialog-tunneling-part-2/ - https://conference.libreoffice.org/2017/the-program-2/oct-12th-thursday/index.html - Tunneling dialogs to LibreOffice Online Kendy's talk at FOSDEM 2018: - https://archive.fosdem.org/2018/schedule/event/ode_dialog/