In fullscreen, when the Minecraft window loses focus (such as when switching to another window using alt+tab), the chat box is cleared. If the chat box was opened by pressing /, then only a / will be in the chat box. (This doesn't seem to happen when Minecraft is not in fullscreen mode.)

Steps to reproduce:
1. Open a Minecraft world in fullscreen mode
2. Open the chatbox using t or /, and type something in it
3. Switch to another window with alt+tab, then switch back to Minecraft