This is a problem that emerged Spring 2020, but we only pinpointed it a few weeks ago.
We have a custom visualization suite based on Electron.js that creates multiple windows using Electron's browserWindow() function. We suddenly noticed (Spring 2020) that Shiny-generated plots of all kinds, including Leaflet plots, were appearing approx 30% smaller (in dimensions that we did not specify "100%" on). We have verified with some custom Shiny diagnostic code that Shiny indeed "thinks" the windows are approx 3x larger than they are. When we hack our plot code to force the plots (or maps) to be 3x larger, they fit --- but are of course slower, because Shiny is generating huge plots.
This only happens on the Windoze deployment of our code. When running the identical Electron-based client code on Linux, it sizes the plots perfectly.