Making changes to, e.g. widget/windows and building using "mach build widget/windows" should cause toolkit/library to also be built
The workaround is to simply "mach build widget" instead of "mach build widget/windows" so this isn't really a big deal
This is really bug 869613. I don't think being explicit about all subdirectories is the right way forward.
You need to log in before you can comment on or make changes to this bug.