sometimes, adding an absolute panel to a composite causes the composite and everything within to be drawn in black color on black background, so the whole composite appears black.
In this state, child widgets can still be placed and selected but the only thing thats visible is the border around the selected widget (which contains some white parts).
Restarting eclipse fixes the problem.
I'm running eclipse 3.3 on suse linux 10.3 (32 Bit) with GWT 1.4.60 and GWT Designer built on 04.12.2007.
This one's a little hard to reproduce - it just sometimes happens, but i can't reproduce it at will.
I hope the description is helpful anyway,