VST3 resizable edit windows

From this page: http://www.steinberg.net/en/company/technologies/vst3.html

“VST3 introduces a new approach to plug-in GUIs though window resizing, allowing for extremely flexible use of valuable screen space.”

I tried this with several 3rd party VST3 plugins, and I have no clue how you would resize the GUI window. Is this an option which has to be programmed by the developer, and only certain 3rd party developers choose to take advantage of this option?

I know some synths I own have incrementally resizable GUI windows, but not stepless resizing by dragging (which is what I make of the explanation in the VST3 specs)