Calculate coordinates of border of geom_textbox() in plot ggplot2 (R)

In a simple ggplot2 plot there is a textbox inserted with the function geom_textbox() and my aim is to insert two more geom_textbox(), one on the left side and one on the right side.

Adding the left textbox is easy - but for the right textbox I need the coordinates of the textbox border. And depending of what I will add to my code, there is a "new" coordinate system...

My code:

plot = ggplot() +
        geom_textbox(aes(x=5,y=5,
                         label = "Test",
                         hjust = 0, 
                         vjust = 0.5,
                         colour = "black", fill = "lightgray", orientation = "upright"),
                     width = unit(6, "cm"),
                     height = unit(2, "cm"),
                     box.r = unit(5.5, "pt")) +
        # left textbox
        #geom_textbox(aes(x=5,y=5,
        #                label = "",
        #                hjust = 1, 
        #                vjust = 0.5,
        #                colour = "black", fill = "lightgray", orientation = "upright"),
        #            width = unit(0.5, "cm"),
        #            height = unit(0.5, "cm"),
        #            box.r = unit(0, "pt")) +
        # right textbox
        #geom_textbox(aes(x=5.???,y=5,
        #                label = "",
        #                hjust = 0, 
        #                vjust = 0.5,
        #                colour = "black", fill = "lightgray", orientation = "upright"),
        #            width = unit(0.5, "cm"),
        #            height = unit(0.5, "cm"),
        #            box.r = unit(0, "pt")) +
        geom_point(aes(x=c(5),y=c(5)), color = "black", size = 2) +
        # the following three geom_point() are in the screenshot and I'm missing the correct x or y value
        #geom_point(aes(x=5.???,y=5), color = "red", size = 3) + # x value of the right border of the textbox
        #geom_point(aes(x=5,y=5.???), color = "red", size = 3) + # y value of the upper border of the textbox
        #geom_point(aes(x=5,y=4.???), color = "red", size = 3) + # y value of the lower border of the textbox
        scale_discrete_identity(aesthetics = c("color", "fill", "orientation")) #+
        #coord_fixed(ratio=1)+
        #xlim(4.5,5.5) + ylim(4.5, 5.5)

The screenshot with the 3 red points, I'm interested in the coordinates.

While my research I also crossed ggplot_build(plot)$data[[1]] but I was not able to use it correctly?!

So may thanks in advance for helping me to find a way to calculate the three red points.

It seems like we have to think differently of the geom_textboxes(). They do not have so much of real bounding boxes as more of a fixed box which always stays the same size independent of the axis scaling. We can still achieve what you want using hjust. About Hjust

library(ggplot2)
#install.packages("ggtext")
library(ggtext)

middle_width <- 6
side_width <- 0.5

ggplot() +
  geom_textbox(aes(x=5,y=5,
                   label = "Middle",
                   hjust = 0, 
                   vjust = 0.5,
                   colour = "black", fill = "lightgray", orientation = "upright"),
               width = unit(middle_width, "cm"),
               height = unit(2, "cm"),
               box.r = unit(5.5, "pt")) +
  # Left textbox
  geom_textbox(aes(x=5,y=5,
                   label = "",
                   hjust = 1, 
                   vjust = 0.5,
                   colour = "black", fill = "lightgray", orientation = "upright"),
               width = unit(side_width, "cm"),
               height = unit(0.5, "cm"),
               box.r = unit(0, "pt")) +
  # Right textbox
  geom_textbox(aes(x=5,y=5,
                   label = "",
                   hjust = -1*middle_width/side_width, 
                   vjust = 0.5,
                   colour = "black", fill = "lightgray", orientation = "upright"),
               width = unit(side_width, "cm"),
               height = unit(0.5, "cm"),
               box.r = unit(0, "pt"))  +
  scale_discrete_identity(aesthetics = c("color", "fill", "orientation"))