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title: Rendering diagnostics description: Inspect renderer scheduling, frame output, cell updates, and timing

Rendering diagnostics

Rendering diagnostics show when OpenTUI schedules work, completes frames, changes terminal cells, and records timing values.

Choose the diagnostic

Do not compare these values as if they used one unit or represented one event.

Timestamps and elapsed time

TimeToFirstDrawRenderable stores one performance.now() reading during its first renderSelf() call. The reading uses the runtime performance time origin. It is a timestamp, not an elapsed startup duration.

import { TimeToFirstDrawRenderable, createCliRenderer } from "@opentui/core"

const startupStartedAt = performance.now()
const renderer = await createCliRenderer()

try {
  const firstDraw = new TimeToFirstDrawRenderable(renderer, {
    label: "First draw timestamp",
    precision: 1,
  })

  renderer.root.add(firstDraw)
  await renderer.idle()

  if (firstDraw.runtimeMs !== null) {
    const elapsedStartupMs = firstDraw.runtimeMs - startupStartedAt
    console.log({ elapsedStartupMs })
  }
} finally {
  renderer.destroy()
}

This example subtracts an explicit application start timestamp from the marker timestamp. The result measures elapsed time to this marker's first draw.

runtimeMs is null before the first draw. Later draws keep the first reading. reset() sets it to null and requests a render. The next draw records a new timestamp.

The component-specific constructor options are fg, label, and precision. Their defaults are "#AAAAAA", "Time to first draw", and 2. The constructor also accepts standard renderable options. After construction, use textLabel and decimals to change the label and precision. The renderable constructor and decimals setter normalize precision. React and Solid assign the initial prop directly, so pass an integer from 0 through 100 to those wrappers.

The Core normalization does not cap precision at its upper end. A value above JavaScript's toFixed() limit of 100 throws during drawing.

The rendered line uses ${label}: ${runtimeMs.toFixed(precision)}ms. OpenTUI truncates it at complete grapheme and display-cell boundaries. It does not split a joined emoji or a wide grapheme.

React and Solid export TimeToFirstDraw wrappers with the same component options. Read the TimeToFirstDraw reference for layout defaults, exact setters, precision limits, and framework examples.

Frame identifiers and events

renderer.frameId is a monotonic JavaScript loop identifier. OpenTUI increments it at the start of each renderer loop attempt.

The renderer emits frame after a pass reaches the rendered state:

renderer.on("frame", ({ frameId }) => {
  console.log("completed frame", frameId)
})

A failed, skipped, or backpressured attempt can increment renderer.frameId without emitting frame. Treat the event payload as an identifier, not an elapsed time or a count of changed cells.

Scheduler state

Use scheduler state when a test or application appears not to settle:

const state = renderer.getSchedulerState()
console.log(state)
await renderer.idle()

renderer.idle() waits for these conditions and any renderer feed-idle retry to finish. It also resolves after renderer destruction. It does not require a zero-cell-update frame.

Native render statistics

renderer.getNativeStats() returns the current native snapshot:

const native = renderer.getNativeStats()
console.log(native.nativeFrameCount, native.cellsUpdated)

An unchanged native frame can increment nativeFrameCount while cellsUpdated is 0. A forced repaint can count the full render surface. Cell updates are terminal cells, not bytes, code points, graphemes, or renderable nodes.

Combined renderer statistics

renderer.getStats() combines native statistics with JavaScript values:

Set gatherStats: true at renderer creation, or call renderer.setGatherStats(true), to collect frameTimes. The default sample limit is 300. Set maxStatSamples to change that limit.

renderer.resetStats() clears the JavaScript frame samples and JavaScript frameCount. It does not reset native counters. Disabling collection with setGatherStats(false) clears the JavaScript samples.

Test renderer output

createTestRenderer() exposes the rendered state without a terminal:

import { TextRenderable } from "@opentui/core"
import { createTestRenderer } from "@opentui/core/testing"

const setup = await createTestRenderer({ width: 20, height: 4 })

try {
  setup.renderer.root.add(new TextRenderable(setup.renderer, { content: "Ready" }))
  await setup.waitForFrame((frame) => frame.includes("Ready"))
  console.log(setup.captureCharFrame())
  console.log(setup.captureSpans())
  console.log(setup.getNativeStats())
  await setup.waitForVisualIdle()
} finally {
  setup.renderer.destroy()
}

captureCharFrame() returns decoded character cells. captureSpans() preserves dimensions, cursor coordinates, colors, base attributes, and span widths. Span capture is lossy for full grapheme clustering and hyperlink IDs. Read Rendering pipeline for those limits.

waitForVisualIdle() returns when the scheduler has no work. While work continues, it waits for a configured number of consecutive frames with cellsUpdated === 0. The defaults are one quiet frame and a maximum of 20 observed frames. Read Testing for bounds and timeout diagnostics.

On-screen diagnostics

The console overlay shows captured application logs. It does not report renderer counters unless your application logs them.

The renderer also has a native statistics overlay:

import { DebugOverlayCorner } from "@opentui/core"

renderer.configureDebugOverlay({
  enabled: true,
  corner: DebugOverlayCorner.bottomRight,
})

Use renderer.toggleDebugOverlay() to change visibility. OTUI_SHOW_STATS=true enables it when the renderer starts.

Buffer and input diagnostics

renderer.dumpBuffers(timestamp?) writes current, next, and output dumps under buffer_dump/. renderer.dumpOutputBuffer(timestamp?) writes only the latest output dump. Both use Date.now() when you omit the timestamp.

Use these environment diagnostics for a bounded investigation:

OTUI_NO_NATIVE_RENDER does not prevent all terminal output. Split-footer output handling can still write ANSI sequences. Input and output captures can contain application data. Remove these settings after the investigation.

Read Environment variables for exact parsing and activation timing. Read the rendering pipeline for layout, cells, diffing, and image protocol behavior.

Next

• TimeToFirstDraw is the exact component reference.
• Testing covers frame capture and visual-idle waits.
• Console overlay covers captured application logs.
• API and symbol index lists the renderer and testing symbols.