Seems to be a bug which creeped into Avenue. We'll see what we can do.
What you could do meanwhile to place sources, would be selecting the empty clip slot by clicking on it, and then click on the clip panel to bring it into the foreground.
Then drag and drop the source to the clip panel.
This should place the Source to the selected clip slot:
- drop.gif (697.42 KiB) Viewed 2036 times