setProperty updated an existing entry in place, so on
style="overflow: hidden; overflow-y: scroll" a later
setProperty("overflow", "hidden") left overflow-y winning, and
removeProperty("overflow") left it behind. Now that the cascade folds
overflow, that showed up as a stale scroll container.
The declaration object stores overflow the way the CSSOM does for every
shorthand: set and parse expand it into overflow-x and overflow-y,
remove drops both, and reading or serializing recombines them when both
are present with the same priority. It is the one shorthand handled this
way because it is the one whose longhands StyleManager tracks.
Adrià Arrufat
b0f6066be7