I have the same issue after the upgrade to R 4.0.0 and RStudio 1.2.5042, but with tmap and mapview (which rely on leaflet, I believe). The maintainer of tmap has concluded that it is an RStudio issue (see comments on his github).
I've tried tmap's view mode from outside RStudio, using the standard console, and it brings up a browser as expected. This is consistent with JanR's experience on exporting to html.
Session info:
R version 4.0.0 (2020-04-24)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18362)
Matrix products: default
locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252 LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] tmap_3.0 mapview_2.7.8 sf_0.9-2
loaded via a namespace (and not attached):
[1] Rcpp_1.0.4.6 compiler_4.0.0 RColorBrewer_1.1-2 base64enc_0.1-3 class_7.3-16 tools_4.0.0
[7] digest_0.6.25 satellite_1.0.2 lifecycle_0.2.0 lattice_0.20-41 viridisLite_0.3.0 png_0.1-7
[13] rlang_0.4.5 DBI_1.1.0 crosstalk_1.1.0.1 parallel_4.0.0 e1071_1.7-3 raster_3.1-5
[19] htmlwidgets_1.5.1 stats4_4.0.0 classInt_0.4-3 leaflet_2.0.3 grid_4.0.0 webshot_0.5.2
[25] R6_2.4.1 XML_3.99-0.3 sp_1.4-1 magrittr_1.5 tmaptools_3.0 scales_1.1.0
[31] codetools_0.2-16 stars_0.4-1 htmltools_0.4.0 leafsync_0.1.0 units_0.6-6 dichromat_2.0-0
[37] abind_1.4-5 colorspace_1.4-1 KernSmooth_2.23-16 munsell_0.5.0 lwgeom_0.2-3 leafem_0.1.1