ggplotly doesn't handle group aesthetic of geom_ribbon

When using geom_ribbon with only the group aesthetic, ggplotly seems to simply merge everything. Semitransparent ribbons, which should appear darker in the overlap region, are therefore indistinguishable.

reprex:

x <- 1:10
ymin1 <- x - 1
ymax1 <- x + 1
ymin2 <- x
ymax2 <- x + 2
p <- ggplot(data.frame(group=rep(c(1,2), each=10), x=rep(x, 2), ymin=c(ymin1, ymin2), ymax=c(ymax1, ymax2)), aes(x=x, group=group)) +
    geom_ribbon(aes(ymin=ymin, ymax=ymax), alpha=0.5)
p

^{Created on 2018-09-20 by the reprex package (v0.2.1.9000)}

从提供的 R reprex 开始，比较仅使用 group aesthetic 时 geom_ribbon 的 ggplot2 结果与 ggplotly 结果。跟踪 ggplotly 入口点，并确定两个带状区域为何会合并；当半透明重叠部分像静态图一样明显变暗时，即表示完成。