The implementation of `npm run watch-sass` was trying really hard
to recompile precisely only the Sass that needed to be recompiled, but in
order to do so, it had to spin up several `watchmedo` processes
per theme. These processes would trigger one another sometimes, leading
to infinite recompilation loops.
Rather than figure out all the dependency directions and messing with
`watchmedo`, I've opted to simplify the script to invoke a single
`watchmedo` process per theme. A single theme recompiles within
seconds, so I think this is a good compromise, one which makes the
script easier to reason about will help me move pass this legacy
assets work.
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