It turns out that the Browser-size OverscrollController keeps track of gesture scrolls sent to the renderer. When the ACKs come back as being handled or not, decides whether that gesture scroll should be applied to the Pull-To-Refresh handler.

IMO, a more straightforward way would be to actually use the overscroll messages already sent to the browser to activate the p2r effect. p2r wants to activate only when a gesture hasn't had any scrolling so we would either have to account for that somehow or add a bit of extra information to the overscroll message.

Triage check-in, this is still relevant and will likely be more so when we undertake scroll unification.