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Theis was closed because the form layout is doing what it was designed to do. The anchors you set are relative to the entire container, not to what is remaining in that container. Your anchor: '100% 100%' setting for the tabpanel means the height of that component fills 100% of the container's entire width and height, not 100% of what remains available at the time the anchor is set.
I'll get an item opened for the designer team to look at the flex for vbox, sounds like an issue there.