Agreed. This has been happening to me for a few years now using PL/SQL Developer. VERY annoying, and a productivity killer - once it happens you cannot minimize PL/SQL developer, which makes it difficult to work on other windows/apps on the same monitor.
The only way to fix it is to close and restart, which is another productivity killer (I run my dev PC for weeks without restarting, and keep apps open configured a certain way for weeks as well, it takes a while to re-setup).
The above described process (1-3) will recreate it, I can confirm.
PLEASE ACKNOWLEDGE AND FIX!