ggplotly only displays one legend

While ggplot produces legends for each aesthetic (other than x & y), after calling ggplotly on the result, only one legend remains (although both titles display).

library(tidyverse)
library(gapminder)
library(plotly)

p <- gapminder %>%
  filter(year == 2007) %>%
  mutate(pop_m = pop / 1000000) %>%
  ggplot(aes(x = gdpPercap, y = lifeExp, colour = continent, size = pop_m, text = country)) +
  geom_point() +
  scale_x_log10(label = scales::comma) +
  scale_size_continuous(label = scales::comma, breaks = c(0, 1, 10, 100, 1000), range = c(1, 10)) +
  labs(x = "GDP per capita ($)", y = "Life expectancy (years)", size = "Population, millions", colour = "Continent")

p

ggplotly(p, tooltip = "text")

I don't think this "disappearing legend" problem is a duplicate amongst the issues I've reviewed, but there is some discussion of this on StackOverflow.

Start by running the reproducible R example in the issue and compare the ggplot and ggplotly outputs, including the colour and size legends. Trace the ggplotly conversion path responsible for legends; done means both legends remain visible with their titles and scales after conversion.