chris wrote:I think that's how I tried to do it originally (a long time ago) and got in a state and gave up. I will try it again.
I thought it might be easier to do something which draws over the entire screen the way WB does, maybe with a transparent layer. I don't actually need to transcend window borders.
It's a shame WB (or, better, Intuition) doesn't expose this sort of functionality, along with the icon dragging visual imagery.
Workbench isn't drawing the box on the screen; it's drawing it on a background window. If you uncheck the menu item Workbench/Backdrop? you will see that you can't draw the box on the screen; only in the window.
I found an example in my OS3 developer CD's called freedraw (with source) that let's you draw a non-animated rubberbanding box in a window. There is an older version of the program on Aminet that has the source too.
Dopus5 does the same animated box as Workbench. It looks to me like it might be an animated BOB. The Dopus5 sources are at SourceForge but it could be difficult to track down the correct functions in that massive amount of code (backdrop_drag.c & drag_routines.c ??).