The specification of JIF states:

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Generally, you add JInternalFrames to a JDesktopPane. The UI delegates the look-and-feel-specific actions to the DesktopManager object maintained by the JDesktopPane.
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In some cases JIFs work perfectly fine in other containers (JPanel, etc.). But in some cases, especially when lightweight and heavyweight components are mixed in a component hierarchy, that is essential to put JIFs into the DesktopPane since the latter provides some additional support (like validating the component hierarchy, etc.) which the mixing feature relies upon. Therefore, it is suggested to extend the specification and additionally state:

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If a JIF object is added to a container other than JDP, certain features or capabilities may behave incorrectly. This includes but is not limited to the behavior when lightweight and heavyweight components are mixed in a component hierarchy (see {@link Component}): some heavyweight components may be displayed incorrectly.
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