linewidth of panel.border and axis.line not identical

[jpquast]

I came across an inconsistency in the line thickness of the panel.border and axis.line. I am not sure if it is a bug or a user error, but I couldn't really make sense of it.
If I set the line width with the linewidth argument the resulting lines are different even if the same value is supplied. The documentation of element_rect() and element_line() says the unit should be "mm". I am not sure which of the two is correct.

Help would be really appreciated! Thanks a lot in advance!

library(ggplot2)

ggplot(data = data.frame(x = 0, y = 0), aes(x = x, y = y)) +
  geom_blank() + 
  theme_minimal() + 
  theme(
    panel.border = element_rect(colour = "black", fill = NA, linewidth = 10),
    axis.line = element_line(colour = "black", linewidth = 10),
    axis.text = element_blank(),
    axis.title = element_blank(),
    panel.grid = element_blank()
  ) 

^{Created on 2024-08-25 with reprex v2.1.1}

[clauswilke]

Half of the panel border is cut off by clipping.

library(ggplot2)

p <- ggplot(data = data.frame(x = 0, y = 0), aes(x = x, y = y)) +
  geom_blank() + 
  theme_minimal() + 
  theme(
    panel.border = element_rect(colour = "black", fill = NA, linewidth = 10),
    axis.line = element_line(colour = "black", linewidth = 10),
    axis.text = element_blank(),
    axis.title = element_blank(),
    panel.grid = element_blank()
  )

p

p + coord_cartesian(clip = "off")

^{Created on 2024-08-25 with reprex v2.0.2}