* This is primarily a service method for ports of widget tool kits
that require single-threaded GUI. DispatchExternalMessage() calls
DispatchMessage(), but also sets fLastMessage, so that
[Detach]CurrentMessage() work correctly. This allows to detach a
message in DispatchMessage() when called from the window thread,
add it to a global queue, and later process the queued messages in
a different thread that calls DispatchExternalMessage().
* BLooper/BWindow: Make sure fLastMessage is accessed only when locked.
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