The bug

I wanted to play around with the new 17w43b snapshot features, when my friend called me on Discord. My minecraft window was in fullscreen so I had to use a keyboard shortcut Alt + Tab to change windows. I picked up the call, and using the same shortcut got back into minecraft. However, (I was flying in gamemode creative) I saw that all of the chunks disappeared. When I left the game menu (Esc), and moved my mouse, all of the chunks previously loaded, reappeared.

How to reproduce

1. Create/Join a world in 17w34b
2. Make sure Minecraft is in fullscreen mode (F11)
3. Press ESC to go to the menu
4. Use the Alt + Tab shortcut to change windows twice
5. Watch, as the chunks disappear.

Note

If you're flying while executing the bug, no chunks will be there, but if you're on ground or little ways from the ground, you will see the chunk you're on.