I believe this patch should fix the backfill issue, but I don't know for sure as I'm not sure how to test it on try.
Bug 1665312 Comment 3 Edit History
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I believe this patch should fix the backfill issue, but I don't know for sure as I'm not sure how to test it on try. I was able to confirm that without this patch, `mochitest-media` does not exist in the full taskgraph, and with this patch it does. I also confirmed that the backfill action is using the full-task-graph to find the task definition, and then adjusting the `test-manifests` key. So I'm *pretty sure* this will work.