pqFlatTreeView hides collapsed items from pipeline
pqFlatTreeView have a major bug with correctly displaying pipeline in certain situations. Video explains a situation and how to reproduce it, some explanation will follow:
I have noticed, that if an item travels between tree levels, if "problematic" item was once expanded manually, it does creates tree items in treeview internals and when it becomes a "toplevel" it rolls out kind of successfully.
Here is the place, where item becomes visually unexpandable: https://gitlab.kitware.com/paraview/paraview/-/blob/master/Qt/Widgets/pqFlatTreeView.cxx#L1384
Not sure if this is correct approach, but pqFlatTreeView is quite complex, it will need a time to understand fully how it functions.
I have to say that pipeline model itself seem intact, because disappeared filters are actually still there, we can see it on rendering view.
Here is a file with a state from video: pqFlatTreeView_bug.pvsm
ParaView versions affected: 5.7-5.9rc1, maybe even earlier versions too.