Currently, building gtk3 against stable cairo results in hidpi support (scaling) being silently disabled.
What other (non-rolling) distributions have been doing is taking a snapshot of cairo HEAD and shipping that.
I'm not well-versed in Debian policies, I'm just coming here to point this out, as it does not seem to be a well-advertised point (nor is it encountered too often - yet).
Obviously the best answer to this would be to see a new stable release of cairo, against which a new gtk3 could be built, but who knows when that might occur (who is best to apply pressure in that direction?), and perhaps there could be some alternate way out of this issue.