qt5 applications running under kde4 appear to have a zero PM_MaximumDragDistance

in for example linguist and qtcreator built with qt5, the scroll bars have the most undesirable behavior of immediately jumping back when the mouse pointer leaves the drag handle.
this behavior does not manifest when i do that in an xfce session, for example.

i suspect that this is somehow related to QKdeTheme being instantiated, but by just reading the code i didn't find a place where PM_MaximumDragDistance/MaximumScrollBarDragDistance could be adversely set. maybe there is some subtle casting problem, possibly involving QVariant.