Business Rules: Unit 1 — Core Editor Engine

BR-01: Paragraph-Level Delayed Rendering

Rule	Description
BR-01.1	When the cursor is inside a structural block, that block MUST display raw Markdown syntax with syntax highlighting
BR-01.2	When the cursor leaves a block, that block MUST render as styled output (formatted text, no visible syntax characters)
BR-01.3	Block activation MUST follow ActivationScope rules (see domain-entities.md)
BR-01.4	Only ONE block activation group may be active at a time
BR-01.5	Switching active blocks MUST re-render both the deactivated and newly activated blocks

Active: Show # prefix characters, plain text, no size change
Inactive: Hide # prefix, render text with heading font size (h1 largest → h6 smallest), bold weight

Active: Show full [text](url) syntax
Inactive: Show “text” as styled link (blue, underline). Cmd+click opens URL in default browser.

Active: Show ![alt](src) syntax
Inactive: Display the image inline (loaded from file path). If image not found, show “Image not found” placeholder with alt text.

Active (wholeBlock scope): Show opening/closing ` ``` ` fences + language tag + raw content in monospace
Inactive: Hide fences, render content in monospace with syntax highlighting (via Highlightr if language specified). Show language label.

Active (currentItem): Show - or * prefix for the active item
Inactive: Replace prefix with bullet marker (•), proper indentation for nesting

Active (currentItem): Show - [ ] or - [x] prefix
Inactive: Replace with interactive checkbox widget. Click toggles [ ] ↔ [x] in source.

Active (wholeBlock scope): Show raw pipe | syntax for all rows and separator
Inactive: Render as formatted table with borders, aligned columns, header row styled bold

Rule	Description
BR-03.1	During IME composition (marked text), the parsing pipeline MUST pause — no re-parse or active block switch
BR-03.2	During IME composition, the active block MUST remain in raw syntax mode (no rendering switch)
BR-03.3	When IME composition commits (marked text finalizes), immediately resume the parse pipeline
BR-03.4	IME composition state is detected via `NSTextInputClient.hasMarkedText()`

Rule	Description
BR-04.1	LaTeX math ( $...$ , `$$...$$`) MUST be rendered as a fenced code block with language `math`
BR-04.2	Mermaid diagrams ( ``mermaid `) are already fenced code blocks — render with language label “mermaid”, no special rendering
BR-04.3	Any fenced code block with an unrecognized language MUST render as plain monospace with the language label displayed
BR-04.4	YAML frontmatter (`---` delimited at document start) MUST be rendered as a code block with language `yaml`

Rule	Description
BR-05.1	Full document re-parse on every text change (after debounce)
BR-05.2	Parse MUST run off main thread (background actor)
BR-05.3	Parse results tagged with documentRevision; stale results (revision < current) MUST be discarded
BR-05.4	Debounce: 50ms after last keystroke before triggering parse
BR-05.5	Diff new parse result against current model; only re-render changed blocks
BR-05.6	RenderCache MUST be invalidated when appearance (light/dark) changes or font settings change

Shortcut	Action	Implementation
Cmd+B	Toggle bold on selection	Wrap/unwrap with `**`
Cmd+I	Toggle italic on selection	Wrap/unwrap with `*`
Cmd+K	Insert/edit link	Wrap with `[]()`, place cursor in URL
Cmd+1 through Cmd+6	Set heading level	Replace/add `#` prefix on current line
Cmd+Shift+K	Toggle inline code	Wrap/unwrap with `

Toggle logic: If selection is already wrapped with the formatting delimiter, remove the delimiters. If not, add them.

When a block is active (showing raw syntax), apply subtle coloring to syntax characters:

Use NSColor.secondaryLabelColor, NSColor.linkColor, and system accent colors for automatic light/dark adaptation.