leaving or changing windows deletes the command block writing

Put the summary of the bug you're having here

What I expected to happen was...:
I wrote a command in the gui box the writing should stay in the menu

What actually happened was...:
when I change window size to fullscreen while I was writing commands
it disappeared out of command block gui

Steps to Reproduce:
1. write something in command block gui
2. change screen size
3.poof it disappeared! and nothing in the gui