If that seems confusing, just go play with it. Odd things happen when you click on a round surface like a stud. Since the drag direction is figured out by the slope where you first clicked, you could be dragging in a diagonal direction. Fortunately, the part will only move by steps and always aligns with the general grid. Right now, there are no rules against dragging pieces inside other pieces.
Transparency seems to stop working if memory gets tight, like with big models.
If you open the same model twice, the second window might not show transparency. BrickDraw3D supports plug-in renderers. Multi-file models are supported, but not MPD files. Simple things like NAME fields are recognized. Other comments just show up in the text area.