SQL Developer Version 21.4.2.018
Java(TM) Platform 11.0.14.1
Oracle IDE 21.4.2.018.1706
OS: Ubuntu 21.10
It is the first time I meet this issue, and in older versions, I have never met it.
I tried resizing the window, and no use.
It seems similar issue has been mentioned before: 4.1EA2 Text cursor goes missing in SQL Worksheet