The Custom Ink Rendering Engine

commitUpdate(
  node: DOMElement,
  _type: ElementNames,
  oldProps: Props,
  newProps: Props,
): void {
  const props = diff(oldProps, newProps)
  const style = diff(oldProps['style'] as Styles, newProps['style'] as Styles)

  if (props) {
    for (const [key, value] of Object.entries(props)) {
      // Event handlers stored separately — never mark dirty
      // (handler identity changes don't affect pixels)
      if (EVENT_HANDLER_PROPS.has(key)) {
        setEventHandler(node, key, value); continue
      }
      setAttribute(node, key, value)   // marks dirty
    }
  }
  // Apply only the changed style keys to the Yoga node
  if (style) applyStyles(node.yogaNode, style)
}

Event handlers are deliberately stored in node._eventHandlers — separate from node.attributes. This means that if a parent component re-renders and passes a new callback reference with the same behavior, the reconciler skips marking dirty. No repaint from a re-created () => handleKey(key).

Every mutation that affects rendering calls markDirty(node). This walks up the parent chain, setting node.dirty = true on every ancestor. The renderer starts at the root and descends. A dirty ancestor forces the renderer to descend past it (no blit), but clean subtrees can still be blitted even inside a dirty ancestor — only the path from root to the changed node is re-rendered.

export type LayoutNode = {
  // Tree manipulation
  insertChild(child: LayoutNode, index: number): void
  removeChild(child: LayoutNode): void

  // Trigger flexbox computation
  calculateLayout(width?: number, height?: number): void
  setMeasureFunc(fn: LayoutMeasureFunc): void

  // Read results (only valid after calculateLayout)
  getComputedLeft(): number
  getComputedTop(): number
  getComputedWidth(): number
  getComputedHeight(): number

  // Style setters (each corresponds to a CSS Flexbox property)
  setFlexDirection(dir: LayoutFlexDirection): void
  setPositionType(type: LayoutPositionType): void
  setOverflow(overflow: LayoutOverflow): void

  // WASM memory management
  free(): void
  freeRecursive(): void
}

The LayoutNode interface decouples the rest of the codebase from Yoga's WASM bindings. Every call to setWidth, setPadding, etc. goes through this interface, so the underlying engine could be swapped — or mocked in tests — without touching the renderer or reconciler.

// Calculate yoga index BEFORE modifying childNodes.
// DOM index ≠ yoga index: ink-progress, ink-link, ink-virtual-text
// have no yogaNodes, so their slots don't exist in the yoga tree.
let yogaIndex = 0
if (newChildNode.yogaNode && node.yogaNode) {
  for (let i = 0; i < index; i++) {
    if (node.childNodes[i]?.yogaNode) {
      yogaIndex++
    }
  }
}

Because some node types (ink-virtual-text, ink-link, ink-progress) don't have Yoga nodes, a DOM childNodes array of length 5 might correspond to a Yoga child count of 2. Any time a node is inserted or removed, the code must manually count how many preceding siblings actually have Yoga nodes to find the correct insertion index in the Yoga tree.

When a scroll box's scrollTop changes and nothing else moves, the renderer emits a ScrollHint — {top, bottom, delta}. The diff layer in log-update.ts can then emit a DECSTBM + SU/SD sequence: a hardware scroll that tells the terminal to physically shift row data, instead of rewriting every changed cell. This is the difference between O(1) and O(rows) escape sequence output for a pure scroll frame.

The pendingScrollDelta field is drained at most innerHeight - 1 rows per frame so the DECSTBM hint can still fire. Fast flicks accumulate in pendingScrollDelta and drain over multiple frames, creating the smooth deceleration effect. The drainAdaptive function handles xterm.js (VS Code), where slow clicks should be instant but fast flicks should stutter slightly. The drainProportional function handles native terminals with proportional wheel events.

intern(styles: AnsiCode[]): number {
  const key = styles.length === 0
    ? ''
    : styles.map(s => s.code).join('\0')
  let id = this.ids.get(key)
  if (id === undefined) {
    const rawId = this.styles.length
    this.styles.push(styles)
    // Bit 0 = "visible on space" (background, inverse, underline, etc.)
    // Even IDs = fg-only styles.  Odd IDs = styles that paint space cells.
    // diffEach uses a single bitmask check to skip invisible spaces.
    id = (rawId << 1) |
      (styles.length > 0 && hasVisibleSpaceEffect(styles) ? 1 : 0)
    this.ids.set(key, id)
  }
  return id
}

The transition(fromId, toId) method is equally clever: it pre-computes and caches the ANSI escape string to transition from one style to another. After the first call for any given pair, subsequent calls return a cached string with zero allocations. For a terminal with 200 columns and a syntax-highlighted response, there might be 10–20 unique style transitions in a line — all cached after the first frame that line appears.

The withInverse, withCurrentMatch, and withSelectionBg methods compose styles non-destructively for the selection overlay and search highlighting — each with its own cache keyed by baseId.

CJK characters, emoji, and other wide characters occupy 2 terminal columns. The screen buffer tracks this with four CellWidth values: Narrow (1 col), Wide (the actual char, marks 2 cols), SpacerTail (the second column of a wide char — do not render), and SpacerHead (a wide char that soft-wrapped to the next line, padding the end of the current line).

When setCellAt writes a Narrow character over a position that previously held a Wide character, it must also clear the ghost SpacerTail at x+1. If it doesn't, the diff will see the SpacerTail as unchanged and skip it — leaving a stale character visible at x+1.

// const enum is inlined at compile time — no runtime object, no property access
export const enum CellWidth {
  Narrow    = 0,  // standard single-column character
  Wide      = 1,  // CJK/emoji — actual char, 2 visual cols
  SpacerTail= 2,  // second col of a wide char — skip in renderer
  SpacerHead= 3,  // wide char at line end that continues on next row
}