The game itself

Tools ▸ Test Map needs the installed game rather than its data: the desktop build's main process looks in the same places the archive search does for StarCraft.exe (x86_64\ and x86\ for Remastered, the folder itself for 1.16) and for a Maps folder, writes the map into Maps\scmJS\ and starts the executable (open -a on macOS, Wine on Linux); a folder picked in the dialog is searched first and remembered in Preferences. A browser has no such reach: Chrome and Edge write into a folder picked once through the File System Access API, whose handle is kept in IndexedDB (services/handleStore.ts, shared with Open Recent), and other browsers download the file.