How to always keep a specific layer in the background in Mapview

I came across a problem when I was trying to visualize polygons using Simple Features and Mapview.

I've created two rectangles, a and b and put them on Mapview on two layers. My problem is that if I select b before a, the b rectangle will be covered by a and will not be properly shown.

Is there any way I could always keep a in the background, so that b is always shown when selected?

library(mapview)
library(sf)

a<-rbind(c(0,0), c(15, 0), c(15, 5), c(0, 5), c(0, 0))
a_polygon<-st_polygon(list(a))

b<-rbind(c(5,1), c(10, 1), c(10, 4), c(5, 4), c(5, 1))
b_polygon<-st_polygon(list(b))


mapview(list(st_sfc(a_polygon), st_sfc(b_polygon)), col.regions = list('blue', 'yellow'), layer.name =c('a', 'b'))

You can use the alpha.regions argument to allow layers to be somewhat see-through.

mapview(a_polygon, col.regions = 'blue', alpha.regions = .6) +
  mapview(b_polygon, col.regions = 'yellow', alpha.regions = .6)

The alpha.regions can be set from 0(not visible) to 1(opaque).This is what it looks like with 'b' selected first, and then 'a' with alpha for both at .6: