Strictly speaking it's not Swarming itself, it's purely recipe work.

We're also considering compressing consecutive green steps (click to expand), which would solve this problem indirectly. I do like the idea of having the triggers as nested steps but I'm less sure how doable that is.

It seem very dooable from what I can see?

I actually have a proof of concept that does a similar thing for the isolate steps (once we use exparchive everywhere, we get an individual isolate step per task in the same way we get individual trigger steps).

This is also in the "buildbot UI is not designed for the things we use it for" problem space, but I think this can be done entirely in the swarming and/or chromium module(s) for now.

 Issue 614760  has been merged into this issue.

This is working on the buildbot pages but not on milo -- all the trigger steps are hidden under the "test_pre_run" step.

Compare 
https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/61809
to
https://luci-milo.appspot.com/buildbot/chromium.linux/Linux%20Tests/61809

It seems Milo doesn't support rendering of the nested steps as being nested. Assigning to the milo team to figure out what they should do.

We have the same issue in Fuchsia. Milo renders nested steps as inline, not as collapsible same as Buildbot did. This was originally implemented as a workaround in 656212, but we'd like to have a proper support for collapsible nested steps.

I'm putting this on the radar for high-priority migration bugs.