Required hardware configuration is a laptop connected to a dock with extra screen.
Steps to reproduce the issue:
Basically, the issue comes from the very poor additionnal screen management in Windows, but most other programs I use like Atom for instance handle this, probably by resetting the window location on startup.
@Overdrivr Yes, this is an issue since we memorize the screen position on quit. But if an extended display is disconnected. It will open at the recorded position. You can temporaly overide this by deleting the electron-config cache.
For those like me that went hunting around, the electron-config cache is at AppData/Roaming/Hyper/config.json. Deleting it does bring back the window.
Alternately, you should be able to move the window around using the normal windows window moving keys (windows+left, windows+right, and windows+up). That should get it back on screen pretty easily.
@MartyGentillon tried this, but Hyper doesn't seem to respond. Also tried moving it to a new virtual desktop.
What worked for me was actually adjusting the "scaling" on my laptop display from 125% (preset) to 100%, something about this made Hyper open off-screen on the external monitor.
Will be fixed by: https://github.com/zeit/hyper/pull/2459
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Alternately, you should be able to move the window around using the normal windows window moving keys (windows+left, windows+right, and windows+up). That should get it back on screen pretty easily.