As the OP I'm okay with wont fix. I've patched the Homebrew formula (with the version_static hack mentioned above, only at build time) and no one's mad at me so far.
Switch from browser action to page action
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Just a suggestion, but according to https://developer.chrome.com/extensions/browserAction:
> Don't use browser actions for features that make sense for only a few pages. Use page actions instead.
Page action apparently makes more sense here because this extension only works on GitHub repo pages.
Using page action also has the additional benefit that within Opera (and possibly other browsers), the icon is entirely hidden when the action isn't activated (which would otherwise show up at the right end of the address bar), reducing unnecessary clutter.
Also, please let me know if you want to update `build/bundle.js` yourself in a separate commit.
This is by design. `SYSCONFDIR` is set to `#{etc}`, that is, `#{HOMEBREW_PREFIX}/etc` instead of `#{prefix}/etc` (see [Formula Cookbook](https://github.com/Homebrew/brew/blob/master/docs/Formula-Cookbook.md#variables-for-directory-locations) for details), so that modifications to config files in `$(brew --prefix)/etc/` aren't lost upon upgrade. This is how most people expect their config files to behave.
> $homebrew_D/Cellar/dmd/2.071.1/bin/dmd
> $homebrew_D/Cellar/dmd/2.072.1/bin/dmd
Unfortunately, Homebrew isn't designed to be completely isolated version sandboxes. Different versions use the same system configurations, just like they would use the same user configurations.
What you could do is to have multiple Homebrew installations.