Context I want to use the ContentEditable attribut instead of classic inputs. It sounds like a duplicate of Affordance of Edit In Place functionality, but actually I would like to avoid the changing ...
The standard way to sinalize the ability to resize is with the operating system's own mouse pointer, the double-pointed arrow in the direction you are to resize (horizontal, vertical or diagonal). ...
Besides the obvious: Scrollbar Touch interfaces (especially mobile ones) A sign saying "You can scroll here you know!" This for a desktop application.