Often when I add an announcement that contains external graphics, the announcement pushes the entire first page way down. It doesn't happen always. What material causes Class Creator to allocate this extra blank space?

There is a graphic with hard coded dimensions in the announcement that exceeds the maximum width of the home page. For classic I think it's around 417. There are older posts that give the size. I'm on Responsive now, which is a lot wider.

IOW, it's not Class Creator that is doing this but the browser's interpretation of your data.

It's also possible is a link is made that is too long and Chrome will also push stuff down.

In the Editor you can manually fix the dimensions.