Open the attached form either inside QtDesigner or compile it in example application. The layout on mac is broken (LayoutMac.png):
1) Look at checkboxes - their x position is not aligned correctly
2) Look at groupboxes - their y position is not aligned correctly (regression from qt 4.2)
If you preview it on mac in different style or on linux it looks correctly (LayoutLinux.png).

This has to be fixed - we can't rely on people not to nest layouts/widgets. This is common pattern to use a placeholder in ui file for something which will be added later (e.g. from plugin).

It looks really ugly on mac - take a look at some examples from qtcreator (LayoutCreator1.png - nested checkboxes, LayoutCreator2.png - "Tabs And Indentation" groupbox is nested while "Indent" is not)