FULL PRODUCT VERSION :
jre 1.6 b105

ADDITIONAL OS VERSION INFORMATION :
When iconifying a JInternalFrame the DefaultDesktopManager class removes the JInternalFrame which causes the window handle associated with the frame to become invalid breaking any JNI client code.
So if you have a canvas tied to a movie viewer it casues the movie to be unloaded.

A DESCRIPTION OF THE PROBLEM :
I am using a canvas that has a movie playing in it and the movied is tied to the cavas via a window handle to the drawing surface. The canvas is added to a JPanel which in turn is added to the JInternalFrame.
When the JInternalFrame is inconified via selecting the minimize button the video code indicates gets a WM_DESTROY message and the window handle associated with the drawing surface for the canvas becomes invalid.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Actually, on the java.net developers forum there is an example suppiled by one of the Sun engineers. The link is:

http://forums.java.net/jive/thread.jspa?threadID=25806&tstart=0

Look at the code example Oleg supplied.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The movie to not to stop and unload and to be available when deiconifying the JInternalFrame.
ACTUAL -
I get a error from the movie code indicating the movie was unloaded

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Created an extends of the DefaultDesktopManager that hides the window instead of removing it when iconfying the window. Upon deinconifying it the window is resized to prior location and size.