When you use Alt+Tab in windows, to switch between netbeans and an other 
window, let's say a notepad, while keeping the cursor on a line of code (of 
netbeans), It takes the focus out of code editor and keeps it on 'File menu' 
of the netbeans. Try using Alt+Tab two times, you will see this.

This way does not let you code quickly/comfortably. It must keep the focus on 
where it was before.

I have logged this issue before, somebody closed and said that this was not 
reproducible.