On bug 827976, we lazy-fied the hidden window creation, and moved it to happen after first paint (on _delayedStartup). We want to make it to not be used at all on a normal browsing session, but there's still some remaining work to do, which will be tracked here.
The hidden window will remain supported in the platform, because:
Here's a talos comparison with a hacky patch to remove the hidden window from about:home (bug 1527213), this allowing this bug to be fixed (not including some other test failures that need to be figured out):
all numbers in the sessionrestore and startup_about_home_paint go down by some 0.5 - 2.0% margin, except startup_about_home_paint on windows10-64-qr, which goes up for some strange reason.