When the mouse hovers the applet and thus triggers its expansion but then quickly left the applet area while it's still expanding, the applet never collapses again by itself, requiring a new hover event instead.
As a workaround I reduced the animation duration, but with the default (500 ms) the problem is frequently caused by random, unintentional, hovers of the applet.
When the mouse hovers the applet and thus triggers its expansion but then quickly left the applet area while it's still expanding, the applet never collapses again by itself, requiring a new hover event instead.