I can't find any place where NS_MOUSE_ACTIVATE is actually used, except here http://mxr.mozilla.org/mozilla-central/source/widget/src/windows/nsWindow.cpp#4124 It was added long time ago, 1999.
Or it is used in NS_IS_MOUSE_EVENT macro, but I can't see where/how acceptActivation to be used.
See bug 392188 comment 13. I think we stopped using it when we switched from Carbon to Cocoa.
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