You can test this on the WMF install by searching for "watch". Either "watc" or "watchl" will match for the project "MediaWiki-Watchlist" but "watch" does not.
From one of our resident JS gurus:
>>! In T118539#1802886, @Krinkle wrote:
> Probably because the JavaScript code in Phabricator treats an object as supporting arbitrary keys. `watch` is a non-standard reserved key in Object.prototype in Firefox's JavaScript engine.
>
> The standard reserved keys are all in CamelCase such as `toString`, `valueOf`, `hasOwnProperty`. But Phabricator's script converts the value to lowercase before it searches, so those are fine.
>
> Upstream bug! They need to use `if hasOwnProperty.call( obj, key )` instead of `if obj[key]`.