Reproducible:

• Start PL/SQL Developer (make sure that the window list is empty)
• Add Windows until the last one is partially displayed (eg: a few pixels).
• Notice that the scrollbar is not visible!!! => NOT Correct
• Add another window => scrollbar is activated => Correct (but too late)
• ** Resetting the situation **
• Remove the last 2 windows => scrollbar is gone
• Add another window (it should be partially displayed again) the scrollbar is not activated => NOT Correct
• Resize the list a little => scrollbar is activated => Correct

This issue let you 'lose' 1 window, something that I encounter frequently.
It's only when you resize the window list (a little bit is enough) or create a new window that this window become accessible again.

PL/SQL Developer 11
PL/SQL Developer 12 Beta 10

We'll check it out.

The scrollbar stays on, even when you close windows that should make the scrollbar disappear.
E.g.: open 10 windows (the last partially hidden, no scrollbar). Open window no. 11, the scrollbar is displayed. Close window 1 and 2. The scrollbar remains and only disappears when you touch the scrollbar.

This has been fixed for 12.0.