Which semantic background a row / widget paints behind its content. Default is a no-op
(inherits whatever the viewport already has, typically theme.bg); Subtle paints
theme.bg_subtle so a config can split e.g. a header band from the main content band.
Both resolve against the theme at draw time rather than carrying a literal colour, so
swapping presets gives a consistent hierarchy.
How children are distributed along the stack’s main axis when their constraints don’t fill
the available space. Legacy is ratatui’s default (proportional fill with Fill constraints);
Center / Start / End / SpaceBetween etc. take a narrower set of children and place
them within the parent.
Where the panel’s title sits along its top rule. Only meaningful when the panel has a
title and a visible border (any BorderStyle except None’s absence-of-panel).