Created attachment 7122 Patch During the development of panel 4.13 the labels Enter and Leave for opacity were swapped. This patch just swaps them back to their original place.
Andre Miranda referenced this bugreport in commit 18a9f030d25f4c9d22af1233ab6dff7ec897cf96 Correct enter and leave opacity label positions (Bug #13594) https://git.xfce.org/xfce/xfce4-panel/commit?id=18a9f030d25f4c9d22af1233ab6dff7ec897cf96
Wow, good catch :)