The bug

When accessing any screen that has a text field selected by default (such as the singleplayer / world selection menu) using the space key, a space gets typed into the text field with which you can search for worlds. Thus, only worlds with a name containing a space are shown.

To reproduce

1. Open Minecraft
2. Press Tab, so that the Singleplayer button is highlighted
3. Press Space
4. A space got typed into the search field
5. Select Create new world
6. Press Space
7. A space got typed into the seed text field

If you press Enter instead, this issue does not happen.