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Folding

Four kinds of blocks fold: headings with their sections, alerts, <details> blocks and a language model's reasoning. Headings fold once AreHeadingsFoldable is set; the others fold where the Markdown asks for it.

<mv:MarkdownViewer Source="avares://MyApp/Docs/guide.md" AreHeadingsFoldable="True" />
Block Folds when Starts
Heading AreHeadingsFoldable is set Expanded
Alert It has a fold marker, > [!NOTE]- or > [!NOTE]+ (needs UseExtendedAlertBlocks() or viewer.UseAlertBlocks()) Collapsed with -, expanded with +
Details block Always (<details>, in the default pipeline) Collapsed, unless <details open>
Thinking block Always (UseThinkingBlocks()) Collapsed, except while a streaming session receives it

What every folded block does

  • Click its header (the heading's button, the alert's title, the summary), or focus it with Tab and press Enter or Space, to show or hide it. Screen readers see a button that reports whether the block is expanded.
  • A collapsed block cannot be selected or copied.
  • ScrollToAnchor, and so in-document links and the table of contents, expands the blocks around its target before scrolling to it. The find methods expand them to show a match.
  • In a streaming session, a block the reader expanded or collapsed keeps that state when an update renders it again.

Foldable headings

AreHeadingsFoldable starts each heading with a fold button, as in Obsidian or Notion, for long documents read section by section.

A heading's section is everything after it up to the next heading of the same or a higher level (## folds the blocks below it until the next ## or #), nested sections included. Collapsing a heading hides its section; the heading stays. Expanding it again shows the section as it was, so a nested heading that was collapsed stays collapsed. Every section starts expanded, and a new document (another Markdown, Source or Pipeline) starts with every section expanded. Setting the property renders the document again.

  • The heading's text stays selectable, and its links clickable.
  • Only headings fold. A heading inside a quote, a list item or an alert folds what follows it there, not the rest of the document.
  • ScrollToAnchor and find expand the sections around their target; a heading navigated to keeps its own section as it was.
  • In a streaming session, blocks that arrive after a collapsed heading join its section, and a heading still being written keeps the state the reader gave it.

Styling

Each kind has its classes, with an :expanded pseudo-class while its content shows; see Style classes. Their content is hidden by the control, not by a style. For example, to dim collapsed headings and show their button more discreetly:

<Style Selector=":is(TextBlock).markdown-heading-foldable:not(:expanded)">
  <Setter Property="Opacity" Value="0.8" />
</Style>
<Style Selector=":is(Control).markdown-heading-fold">
  <Setter Property="Opacity" Value="0.6" />
</Style>
<Style Selector=":is(Control).markdown-heading-fold:pointerover">
  <Setter Property="Opacity" Value="1" />
</Style>