PositionStack (and others) expects a numeric, non factor, "group"

PositionStack and similar classes that use collide() and collide2 to sort the data expects "group" to be numeric, not factor.
Some Stats may create a "factor" group column, though not those which might be stacked (i.e. StatContour), the problem is relevant for custom Stat extensions that might update group.
The problem arises because for reverse = TRUE, the "collide" functions use order(..., -data$group) instead of order(..., data$group, decreasing = TRUE), as "-" is not meaningful for factors.
https://github.com/tidyverse/ggplot2/blob/c02c05aa6303e9592e37289d780224a06be5a27e/R/position-collide.R#L43

ggplot(faithfuld, aes(waiting, eruptions, z = density)) + 
  geom_contour(position = "stack")
# warning "-" no meaningful for factors
```

Start in R/position-collide.R around the linked line and inspect how collide() and collide2 order group when reverse is TRUE. Reproduce the supplied geom_contour example, then verify that factor-valued group data no longer produces the warning and is ordered correctly.