Signs do not register input after using Alt+Tab to copypaste text

Steps to reproduce problem:

1. place a hanging sign 
2. enter some text
3. use ctrl+a to select all text and ctrl+c to copy them
4. close the ui, then place a new hanging sign
5. try paste text with ctrl+v,You will find that you can't paste or enter anything.

if problem not occur, repeat 4-6 for a few times
sometimes you may need to remove the first sign and then paste text to the new sign and this problem occurs randomly