> This is a bit tangent, but does anyone know how to block an app from ever deciding to become a file handler on macOS?

The information below does not "block an app" from taking over file handler associations. It may be beneficial on its own and/or provide a starting point for further exploration.

> Every time I install Chrome on my machine (for testing purposes), macOS decides that Chrome is going to be the default file handler for a bunch of file associations, including HTML, WebP, and so on… and I have to figure out which was which for all of the mappings (which is super frustrating).

The following command can display current file extension associations for a user account:

  plutil -convert xml1 \
    ~/Library/Preferences/com.apple.LaunchServices.plist \
    -o -

Also regarding a Chrome OS-X/macOS installation - it installs plists for unconditional background updates. This may not be a desirable feature.