On Thu, 2022-02-17 at 17:32 +0200, Ilmari Lauhakangas wrote: > vcl/source/control/ivctrl.cxx has collectUIInformation with the > parent as the last parameter. It is run in > VerticalTabControl::SetCurPageId > > I set a breakpoint there and indeed 'p GetPageParent()->get_id()' > returns an empty string. > > GetPageParent() is in vcl/inc/verticaltabctrl.hxx and gets a VclVBox > object. > > VclVBox is found in include/vcl/layout.hxx So GetPageParent() is just m_xBox.get() and that m_xBox is the one created in the ctor of VerticalTabControl. If it should have an id then some m_xBox->set_id("whatever") would make that string non-blank. I don't really know if that "Parent" is particularly useful for any purpose? I don't think I see anything equivalent for the "normal" horizontal TabControl, maybe it makes more sense to drop the "Parent" argument instead of giving it an id.