I noticed this just now, setPopupDisplayEnabled seems to be broken on Linux.
I haven't tested other OSes. Not sure when this started happening.
The popup is displayed based on 0,0 position instead of actual plugin pos.
See this to understand: http://i.imgur.com/vrQOl1C.png
When the slider is pressed down this also happens:
JUCE Assertion failure in juce_DropShadowEffect.cpp:48
The code for this UI is here: https://github.com/DISTRHO/DISTRHO/blob/master/ports/arctican-function/source/PluginEditor.cpp