> In linux and I imagine on Windows and macOS, when there exists a vertical OS toolbar the values returned by get-display-size and get-client-size are the same and are insufficient.
The macOS version, at least, is draggable so there can be different widths on different Finder windows at the same time. You can get the info by shelling out to the command line if that's a possibility for you.
$osascript -e 'tell application "Finder" to get the sidebar width of Finder window 1'
or
$osascript -e 'tell application "Finder" to get the sidebar width of front Finder window'
or
$defaults read com.apple.finder | grep SidebarWidth
Note that this gives a list of results but I am not clear on whether they are all default values or values for current windows or, more likely, some of each. The file com.apple.finder is a plist file which is a form of xml so you might even read the file directly into Racket and parse it.
I would not be at all surprised if there are multiple other ways to get the info from the command line.
James