Plasma panels & Latte docks currently have different expectations or wishes about placement of & frames around popups.
R242:6aedda94b27a3ac60176dce960484af3f7787c07 added a heuristic valid at that time to detect a Plasma panel (no mask set), but this heuristic no longer applies due to R120:65367689ea5e6443eeea36398dda0bd48503e1bc (which now sets a mask matching the current Plasma theme). Already before there was an alternative patch proposed (as the original patch/heuristic already has not worked for non-compositing mode): D15821
Citing @mvourlakos :
"
That creates two questions:
# How Plasma popups should be placed correctly when the window sets a mask?
# Will there be any way for Plasma popups to be placed independently of Window geometry OR Window:mask() ? For example based on the item that triggered the popup event
"
Another issue are frames around popups, where Latte dock (edit: )~~always~~ most of the times prefers full-framed windows, while Plasma currently tries to remove borders on sides touching screen edge or panel edges. Some unapplied patch based on above mentioned heuristics is D15814.
Personally, I wonder if there is a conceptual difference between "Plasma panels" and "Latte docks" from a user POV. To me both are principally the same, only allowing different kind of visual gimmicks for user interaction and item layout. In a perfect world Latte would be merged into Plasma for some panel v2.0 option :)