The documentation for NSPopUpButton states:
An NSPopUpButton object uses an NSPopUpButtonCell object to implement its user interface.
I derived a new class from NSPopUpButtonCell which implements drawBorderAndBackgroundWithFrame:inView: to achieve custom drawing. In addition, I derived a new class from NSPopUpButton and use cellClass to use my derived class for it. (i.e. I am not working via interface builder.)
With the advent of macOS 10.14 beta however, this routine is not called anymore, and I observe the "normal" (i.e. non-customized) drawing.
Thanks to @Marc T.'s answer, I (think I) was able to localize the problem to the fact that the cell apparently calls drawBorderAndBackgroundWithFrame:inView: only if drawInteriorWithFrame:inView: is implemented in the derived cell.
I could not find solid documentation on this. Is there any?
If there would be such a change in macOS 10.14 I assume Apple would have announced this on WWDC 2018. A quick check in Interface Builder showed me that nothing has changed so far. Creating a subclass of NSPopUpButtonCell to use it as the class for the popup button cell is still working as expected. What I have to mention is that drawBorderAndBackground is only called if drawInterior is implemented as well. Since I was never using drawBorderAndBackground before, I can not say if this is a change from previous versions to 10.14 or not.
Hope this helps.