When a dock separator has WA_NativeWindow, it becomes very hard to manipulate. This can happen very easily: if any widget in the same window has WA_NativeWindow set, this will propagate to the central widget (unless WA_DontCreateNativeAncestors prevents it), which will make its siblings native (unless AA_DontCreateNativeWidgetSiblings prevents it), which includes any separators created in the main window.
This can be reproduced with a small modification to qtbase/examples/widgets/mainwindows/dockwidgets/mainwindow.cpp: insert "textEdit->setAttribute(Qt::WA_NativeWindow, true);" before the first line of the MainWindow constructor.
This seems to have something to do with the widget mask. Perhaps the mac input code is not respecting WA_MouseNoMask.