Comment 8 for bug 1804691

@Seth, thanks for taking a look. That seems plausible:

commit 38c5b025c8abdcab842587f54f2b64317d47ca34
Author: Seth Hillbrand
Date: Wed Oct 17 17:05:30 2018 -0700

    Ratsnest: iterate only over dirty nets

    Rather than forcing each thread to iterate over the full nets list to
    find the dirty items, we collect the dirty items first before asking the
    threads to address them. This prevents thread spinning. We also
    amortize the cost of calculating on an 8-net per thread basis rather
    than 2.

and about 1 day and 10 commits or so later:

commit e1f30a3b382f91f772483936b589c98b2bb67d6d
Author: Seth Hillbrand
Date: Thu Oct 18 17:11:58 2018 -0700

    pcbnew: Fix single-net connectivity

    Ensure that changes affecting fewer than 4 nets still get updates in the
    ratsnest.

If those are indeed the cause and fix of the bug, then I'm pretty unlucky to have pulled the nightly at just the right time to get that problem to build PCBs with for the last month. :)