I found that the first time I flashed Android L, a curious bug appeared — my home and recents buttons disappeared, and the notification shade refused to slide down. It seems I’m not alone; this bug only appears when upgrading/flashing Android L without wiping. It’s not clear exactly why this happens, but the good thing is that it’s relatively easy to fix.
If you find your Nexus 5 or Nexus 7 behaving this way, a quick ADB shell command will get it working properly again. If you’ve flashed Android L, we’re assuming you’re familiar with how to use ADB, so here’s the command you’ll need:
adb shell am start -n com.google.android.setupwizard/.SetupWizardTestActivity
It seems this isn’t a new fix; Paul O’Brien of Modaco fame figured this out some time ago, when he noticed the same kind of issue appearing when flashing a KitKat OTA update. What this command does is display the Welcome dialog as if you were setting up your phone from scratch. You’ll be asked to pick a language, join Google Now, and then you’ll be taken back to your home screen.
This time, the notification shade will work, but you may or may not see your home and recents buttons. If you don’t, just reboot your device and they’ll be right back where they belong.