For some time, I noticed that the appearance of the Mac OS context menu (right-click menu) and popups changed.
They look like in older OSX versions. In the upper picture, the corners should be rounded; in the lower picture, the blue should be different.
I don't really know why this changed or whether it's an editable setting.
I think that this behavior occurred after updating some Adobe programs (After Effects, Dimension CC, etc.).
My current OSX version is 10.13.6, on a MBP (late 2016, 15").
Best Answer
So after a long time, I found out that the actual behavior is a setting:
System Settings -> Accessibility -> Display -> Reduce Transparency -> On/Off
If you turn Reduce Transparency off, the usual, normal context menu will appear again. I have no idea who or what changed this setting.