Footer does not stick to bottom of the page in fillable layout using page_navbar()

Describe the problem

When using the footer option in page_navbar() with a fillable layout, the footer does not adjust to the changing page size. Instead, it just stays under the UI elements in the location where the bottom of the initial page was.

If you use the fillable = FALSE argument, then the footer is always present right below your last UI element, instead of at the bottom of the page. However, it does stay below all elements, and does adjust as new elements are added.

I coded this up on the shinylive website so that people could play with the fillable = TRUE or fillable = FALSE options and could see the behavior of the footer text.

I just repeated the example text to create a very large UI as the example.
Here is the code that is also shown on the shinylive site.

library(shiny)
library(bslib)

ui <- page_navbar( #<<
  nav_panel("A", tagList("Page A content",
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content", 
                         br(), "Page A content")), #<<
  nav_panel("B", "Page B content"), #<<
  nav_panel("C", "Page C content"), #<<
  title = "App with navbar", #<<
  id = "page",
  footer = "my footer",
  fillable = TRUE#<<
) #<<

server <- function(input, output) {

}

shinyApp(ui = ui, server = server)

https://shinylive.io/r/editor/#code=NobwRAdghgtgpmAXGKAHVA6ASmANGAYwHsIAXOMpMAGwEsAjAJykYE8AKAZwAtaJWAlAB0IdJiw71OY4RBEBXWgAIAPAFolqKAHM4AfWgA3ei3ZKAxCpUilSo3q0Q41dkLABBN7iWkdAGVpOUlcwAAUdOCV3JWIyClIvG1tklNS0lKZ2AW83cN0omJJyMi8lJPSKiszssrCIgtjihLwyiEr2tOqcuvzoxvjS8o6Ortq8yL6igZah4crR3PrJuJKZtrmRxizu8Yap1Zz1jfmtmsXewpXmw+P2hZ6Jy6bBo9vO052lp+mbt-T73bLZ5rP7-D5jL79A6tUHvbYQi5Q64w2EZcHnR5Il6otHwjF7K7YnG2AGQ-bI2aw0mI8lE4nUzG0kHEpQMgnA3709EPdk-FFcvE8oF8ymgtnC6Giv7i76S16omVY5kCs5C2UU+VU7mA9V0nGKpmc-XasmE5XGwU6pVGhUmmlmm1ay2mjn8i2qq2Gt2253211St4Gh3ep0el0izViu2M4MB25B-2R6XR3lylmslMSjXphMRnOZ3Xmn1hv15lm5tPlgvWkNR30xxP5+up7NV5tZvXFz6l1YCGqWazreyOZwhABCgx5Y8L-asSWHUCcLjcAGFJ7sVzPvAOkqRaKRqJEALy1dzoJQAd333DsUGMLFKO-WtAAJkoT24tLpEusAGZEIhyEYd9ahgVglH-QC4EYH9bF-WhqGoKB6EPECABUsAAVQAUSfAQLDnOQIE4aDDGg1QNF-eQIAIPcSHYPhUHkUhvCIZimNIfCQBEEQAF8eOI3h+DPVB2EUEDFG8EjGDI4CT2k2TZDAXiAF0gA

I added the sessionInfo() from the shinylive site, for completeness.

Session Info

R version 4.4.1 (2024-06-14)
Platform: wasm32-unknown-emscripten (32-bit)
Running under: emscripten
Matrix products: default
locale:

[1] en_US.UTF-8 C           en_US.UTF-8 en_US.UTF-8 en_US.UTF-8 C
time zone: America/New_York

tzcode source: internal
attached base packages:

[1] stats     graphics  grDevices utils     datasets  methods   base
other attached packages:

[1] bslib_0.8.0      shiny_1.9.1.8002
loaded via a namespace (and not attached):

[1] digest_0.6.37     later_1.3.2       R6_2.5.1          codetools_0.2-20

[5] httpuv_1.6.6.9000 fastmap_1.2.0     webr_0.4.2        magrittr_2.0.3

[9] cachem_1.1.0      glue_1.7.0        memoise_2.0.1     htmltools_0.5.8.1

[13] lifecycle_1.0.4   promises_1.3.0    cli_3.6.3         xtable_1.8-4

[17] sass_0.4.9        jquerylib_0.1.4   withr_3.0.1       renv_1.0.7

[21] tools_4.4.1       mime_0.12         Rcpp_1.0.13       fs_1.6.3.9000

[25] rlang_1.1.4       jsonlite_1.8.8

Reproduce the supplied app using page_navbar() with fillable = TRUE and compare it with fillable = FALSE, focusing on how the footer responds as the page content changes size. Trace the page_navbar() layout entry point and identify the existing footer behavior. Done means the footer stays at the bottom for the fillable layout while remaining below all content as the page grows.