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Layout( self, theme: Theme, )
Lay out sibling blocks.
Lay out one block.
Lay out a heading across the full width; h1 is centered.
h1
Lay out a code block, folding lines longer than the block.
Fold at cell boundaries without splitting wide characters.
Return the highlight theme MarkdownFence.highlight would pick.
MarkdownFence.highlight
Lay out a block quote: an outer left border and one cell of padding.
Lay out a list: a bullet column beside each item's blocks.
Lay out parsed blocks at a width, following Textual's block CSS.
