GTK+ 3 theming for emoji completion uses border-spacing, which GTK+ 3 lacks
Commit 65bb238a picked the CSS from
master, so now we get errors at runtime about the invalid property name in
gtk-3-24 (I can trigger them by opening the Inspector on any old program), albeit only after regenerating, which that commit had not done.
My vote would go to backporting
border-spacing as it's an extremely useful property, but I don't know if anyone will want to do that. If not, some alternative way of spacing the children will be needed. In my own GTK+ 3 apps, I've done some impressive and/or horrifying things to emulate
border-spacing (albeit verbosely and with constraints on what is possible), but there is probably a nicer, simpler way in this particular case.
Btw, this is only in Adwaita; HC appears to have been skipped from this theming update, not that that is anything really new!