It will flash, there's just a 15 second delay. It will also switch to that conversation and because of this delay it will give you time to finish reading/writing in the active conversation, the switching conversations is needed for the flashing (notice the namechange in taskbarbutton). If you switch off that option (plus > preferences > instant messages > tabbed chats...."auto-activate chat windows with unread messages"), there will be no more autoswitching screens but also no more taskbarflashes for unfocused chats.
You can change the delay if you wish to do so, either manually or with Plus! Tweak (this program changes the registrysettings for you). The regkey you're looking for is GroupAutoSwitchDelay. To download Plus! Tweak or for more information about this regkey see:
Help Registry Tweaks
Edit: you're welcome
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