One design characteristic of our QtWidgets is that they contain a lot of frames and frames inside other frames. This worked well with Oxygen style and its skeuomorphism shadow, less so with Breeze. I first thought this was inheriten with QtWidgets and couldn’t be fixed without much effort. But fortunately, after looking a bit into Qt source codes and in particular in the internals of QDockAreaLayout, I discovered that the engine to draw and style the built-in components of QtWidgets: QStyle has a QStyle::PE_IndicatorDockWidgetResizeHandle primitive which allows drawing separators between detachable docks and similarly there is QStyle::CE_Splitter to paint the separator between elements inside a QSplitter.