I discovered that the issue is not reproducible with the external monitor unplugged, i.e. the trigger seems to be lightdm starting in dual monitor mode. That invalidates the test I did at bf1d6b2c76eda86159519bf5c427b1fa8f51f733 , which I think was with the monitor unplugged. But since e5fa05b96b02f45a91a364d2121451a4da05cd84 is definitely fixed, i.e. lightdm starts correctly even with the monitor plugged in, immediately after confirming the bug in a known-bad kernel, I think we're still on the same range, so we don't need to retest any kernels.
e5fa05b96b02f45 a91a364d2121451 a4da05cd84: fixed
I discovered that the issue is not reproducible with the external monitor unplugged, i.e. the trigger seems to be lightdm starting in dual monitor mode. That invalidates the test I did at bf1d6b2c76eda86 159519bf5c427b1 fa8f51f733 , which I think was with the monitor unplugged. But since e5fa05b96b02f45 a91a364d2121451 a4da05cd84 is definitely fixed, i.e. lightdm starts correctly even with the monitor plugged in, immediately after confirming the bug in a known-bad kernel, I think we're still on the same range, so we don't need to retest any kernels.