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For me (Ubuntu 10.10), double clicking to select a single word in ST2 updates the X selection buffer correctly. Tripple clicking to select a whole line sometimes copies the whole line and at other times just a single word. Selecting by dragging seems to copy only the first 1 or 2 characters. If I make a selection really quickly then it copies the whole lot. I guess ST2 makes the copy a few milliseconds after the selection has started, regardless of whether the mouse is still down or not.