>From Scott Violet in evaluation to 4327621.

KeyboardFocusManager keeps a HashMap that maps from a Window to the
Component that last had focus in the Window. The HashMap never
appears to be cleared, thus resulting in keeping all Windows that are
created from being gc'ed. A simple fix would be to change
Window.dispose to remove the Window from the HashMap. Another option
map to use a WeakRef, but dispose may be enough.