Several internal consumers (e.g. session history, scroll anchoring) have a need to change the visual viewport offset from the main thread. This bug tracks adding an internal API for making such changes. The API can live in nsIPresShell for C++ consumers, with an nsIDOMWindowUtils wrapper for JS consumers. The implementation would send a message to APZ and update the RCD-RSF visual viewport offset, while leaving the layout viewport offset alone (modulo changes needed to keep the layout viewport enclosing the visual viewport).

(In reply to Botond Ballo [:botond] from comment #0) > The implementation would send a message to APZ and update the RCD-RSF visual > viewport offset, while leaving the layout viewport offset alone (modulo > changes needed to keep the layout viewport enclosing the visual viewport). After thinking about this some more, I think it would make more sense to send the update as part of the next transaction, rather than having a separate message. We can add a "visual viewport offset updated" flag to ScrollMetadata, and have APZ accept both the layout and visual offsets sent by the main thread.

Depends on D16141

Pushed by bballo@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/bea2de966d14 Add a mechanism for the main thread to set a visual viewport offset. r=kats https://hg.mozilla.org/integration/autoland/rev/47c259e23cfe Expose the visual scrolling mechanism to internal JS users. r=kats