It's rchat's fading lua.
CHAT_TAB_SHOW_DELAY = 0
CHAT_TAB_HIDE_DELAY = 0
CHAT_FRAME_TAB_SELECTED_MOUSEOVER_ALPHA = 1
CHAT_FRAME_TAB_SELECTED_NOMOUSE_ALPHA = 1
CHAT_FRAME_TAB_ALERTING_MOUSEOVER_ALPHA = 1
CHAT_FRAME_TAB_ALERTING_NOMOUSE_ALPHA = 1
CHAT_FRAME_TAB_NORMAL_MOUSEOVER_ALPHA = 1
CHAT_FRAME_TAB_NORMAL_NOMOUSE_ALPHA = 1
DEFAULT_CHATFRAME_ALPHA = 0
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