This will keep the event from propagating. Accepting a press event will also not result in clicked or pressAndHold being handled.
In Qt 5.10 event propagation changed resulting in FolderView opening both the item context menu and containment context menu. Imho this actually makes sense since we never accepted the mouse event there.

I put an mouse.accepted in FolderView after opening the context menu and no longer get double menus showing up.
However, I'm not sure how delicate this change is. In theory it shouldn't change existing behavior and fixing/changing FV might be the better solution?