check your display settings in system preferences. sometimes (and this goes way back), the mac could be booting with a different screen resolution, THEN reading your settings as it finishes booting...
If this is the same bug as i've noticed; it happens when you're plugged in using an external monitor (at least with HDMI as that's what I have). If i'm not plugged in to an external monitor things are fine. I have reported it to Apple, but either way you should be reporting it.