plotly::subplot with >= 3 rows/cols results in different sized plots for inner/boundary plots

As per title, when making a subplot with >= 3 rows or columns with a margin (default is 0.02 but the issue is easier to see with more plots/bigger margins)

library(magrittr)
p <- plotly::plot_ly(mtcars, x = ~cyl, y = ~mpg, color = ~rownames(.data), type = "bar") %>% plotly::layout(showlegend = FALSE)
plotly::subplot(p, p, p, p, p, p, p, p, p, nrows = 3, margin = 0.1)

There is a PR (#622) which aims to fix it, but the PR also does some extra things like make the margins fixed rather than scaled. I think it's possible to provide a much quicker & simpler fix as an alternative, given that this has been known about since 2016.

I think in a nutshell its due to the function get_domains & it's for loops:
https://github.com/plotly/plotly.R/blob/4bb1e44c5e27898bdf7ac45422564fa5200fb5f5/R/subplots.R#L385-L401
It's those if's that are the problem, can they be removed without causing too much trouble?

Start with R/subplots.R, especially get_domains and the loops around lines 385–401. Run the supplied R example with nrows = 3 and margin = 0.1, then compare the domain sizing for inner and boundary plots. Done means subplot panels have consistent sizes; review PR #622 for related work before proceeding.