I'd swear this used to be a widespread feature, but now I can't find any evidence of it. A table cell contains information that is too wide for the column, so it is displayed in truncated form, and when the mouse is over the cell, a wide box appeared overlaying the table, similar to a tooltip but not a tooltip, to show more of the cell's content.
The thing that makes this fundamentally different from a tooltip is that the extendo-box (as I'm calling it until I hear a better name) would be positioned on top of the cell that it is giving information about, aligned so that the text you were able to see before is still in exactly the same position on the screen. You could just keep reading past the edge of the cell, instead of having to find your place again.
It's probably hard to understand from my description so here's a demo: http://jsfiddle.net/9z2xt/1/ ... try a mouseover on the table cells and watch how the text extends itself without moving.
I added one of these to a project under development, and although I replicated all the behavior I remember (as in the jsfiddle), it didn't turn out to be very helpful. I don't know what to do about alignment of the extended text when the cell is all the way on the right. I'm inclined to get rid of it. But the origin question is bothering me now.
Where did I get this idea from? Was it part of a common widget set like GTK, Qt, Motif? Is it still used in the current version of anything? Or did I really dream it up myself?
I finally spotted one in the wild, on a Windows box. It's in the Resource Monitor.