There has also been discussion about some MDI type interface, although the main developers seem dead against it, mainly on account of that they feel this is something that should be solved using the window manager, not the application.
Please note that GIMP does MDI (multiple document interface) already, it just doesn't folllow the WiW (window in window) approach that some people seem to prefer for obscure reasons.
A good window manager makes the window-in-window concept unnecessary. However if someone wanted to develop such an approach for all the poor users that are forced to use a not so advanced window manager, then it should probably be addressed at the toolkit level.
What Sven is saying, is that it is basically impossible to get Window in Window support working on linux. You can do it, but is sucks a whole, whole lot. On Windows, the best approach will be to add support for Window-in-Window to gtk+ our gui toolkit, as that kind of feature shouldn't be implemented in the gimp.
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