When did they change "Trash" to "Bin" in Mail?
I can tell you that this only fixes the issue temporarily. I have just had it happen again on my 9.7 Pro after setting my wallpaper to still instead of perspective.
Try cycling reduce transparency on and off. That restored normal dock behaviour for me. Not sure if it's temporary yet, it most likely is though. Looks like a graphics driver bug, or memory issue.