[R-pkg-team] Bug#1104454: r-cran-bslib: test fail - Undefined variable: "$transition-collapse-width"

[Rebecca N. Palmer]

Package: r-cran-bslib
Version: 0.9.0+dfsg-2
Severity: serious
Control: affects -1 r-cran-rmarkdown

Since twitter-bootstrap4 4.6.2 added transition-collapse-width, some 
r-cran-bslib and r-cran-rmarkdown tests are failing with

Error in `compile_data(sass_input, options)`: Error: Undefined variable: 
"$transition-collapse-width".
          on line 24:25 of 
../../../../../../usr/share/sass/bootstrap/_transitions.scss
          from line 29:9 of stdin
  >>     @include transition($transition-collapse-width);

I don't yet know why the definition in 
/usr/share/sass/bootstrap/_variables.scss isn't being seen.

(This is blocking r-base + r-cran-testthat but isn't actually their 
fault: r-cran-testthat used to have a bug that made it not always 
recognizably-to-autopkgtest report failures, which has now been fixed.)