>I'm drawing a blank on this feature where like a textbox or some other text control has too much text and by setting this magical control property it looks like a tooltiptext by extending the text display beyond the control?

There is DISPLAYPATH() function doing this, but the tricky thing is to determine the length parameter. For a static textbox one can fiddle around with FONTMETRIC(6,), but for a resizeable column one must do this for any change of column.width .
Also the WIDTH of a textbox compared with FONTMETRIC has odds, there are visible and unvisible borders on the textbox.

And only for columns the

.ControlSource = 'DISPLAYPATH(cur.field,20)'

works.
This is not possible for a normal textbox. You must prepare the shortened string in your cursor, and somehow manage to get the full text in the ToolTipText everytime the record changes.

Or you use something like Dragan suggest to bring a second textbox in foreground for edit / show.