In a nutshell, DF uses a different approach than Windows does when adding the visual indicator that multiple windows are grouped under the same button (which makes it look like overlapped buttons) on its taskbars: Windows "slices" the button but keeps its width, while DF widens it.

This wouldn't be a problem if not for the fact that it causes the "running" indicator (the line the shows up underneath the app's icon when it's open) shows off-centre on the DF taskbars. The image I attached should clarify what I mean.