grafana/k6 · error

emulating media: %w

Error message

emulating media: %w

What it means

Page.EmulateMedia stores media type / color scheme / reduced motion settings and applies them via Emulation.setEmulatedMedia in every frame session. This error means at least one frame session's CDP call failed while iterating under the frame-sessions read lock.

Source

Thrown at internal/js/modules/k6/browser/common/page.go:1076

func (p *Page) DispatchEvent(selector string, typ string, eventInit any, opts *FrameDispatchEventOptions) error {
	p.logger.Debugf("Page:DispatchEvent", "sid:%v selector:%s", p.sessionID(), selector)

	return p.MainFrame().DispatchEvent(selector, typ, eventInit, opts)
}

// EmulateMedia emulates the given media type.
func (p *Page) EmulateMedia(popts *PageEmulateMediaOptions) error {
	p.logger.Debugf("Page:EmulateMedia", "sid:%v", p.sessionID())

	p.mediaType = popts.Media
	p.colorScheme = popts.ColorScheme
	p.reducedMotion = popts.ReducedMotion

	p.frameSessionsMu.RLock()
	for _, fs := range p.frameSessions {
		if err := fs.updateEmulateMedia(); err != nil {
			p.frameSessionsMu.RUnlock()
			return fmt.Errorf("emulating media: %w", err)
		}
	}
	p.frameSessionsMu.RUnlock()

	applySlowMo(p.ctx)

	return nil
}

// EmulateVisionDeficiency activates/deactivates emulation of a vision deficiency.
func (p *Page) EmulateVisionDeficiency(typ string) error {
	p.logger.Debugf("Page:EmulateVisionDeficiency", "sid:%v typ:%s", p.sessionID(), typ)

	validTypes := map[string]emulation.SetEmulatedVisionDeficiencyType{
		"achromatopsia": emulation.SetEmulatedVisionDeficiencyTypeAchromatopsia,
		"blurredVision": emulation.SetEmulatedVisionDeficiencyTypeBlurredVision,
		"deuteranopia":  emulation.SetEmulatedVisionDeficiencyTypeDeuteranopia,
		"none":          emulation.SetEmulatedVisionDeficiencyTypeNone,

View on GitHub (pinned to 93accf6570)

Solutions

1. Call page.emulateMedia before navigation or after a stable load state
2. Avoid calling it concurrently with page.goto() (await each call)
3. Ensure the page is not closing when the call is made
4. Apply emulation at browser-context level (newContext({colorScheme: ...})) for pages with unstable iframes

Example fix

// before
await page.goto('https://example.com/');
await page.emulateMedia({ media: 'print' }); // races with iframe attach

// after
await page.emulateMedia({ media: 'print' });
await page.goto('https://example.com/');

const opts = { media: 'print', colorScheme: 'dark', reducedMotion: 'reduce' };
if (!['print','screen','null',''].includes(opts.media)) throw new Error('bad media');

try {
  await page.emulateMedia(opts);
} catch (e) {
  if (/emulating media/.test(e.message)) { await page.waitForLoadState(); await page.emulateMedia(opts); }
  else throw e;
}

When it happens

Trigger: page.emulateMedia({ media: 'print', colorScheme: 'dark', reducedMotion: 'reduce' }) while iframes are attaching/detaching, during navigation, or on a page whose session is closing.

Common situations: Emulating print media on pages with ad/analytics iframes that churn; calling emulateMedia concurrently with page.goto; page already closing at iteration end.

Related errors

• getting iframe frame: %w
• getting remote node %q: %w
• internal error while initializing frame %T: %w
• internal error while updating emulated media: %w
• getting frame owner: %w

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/84288f341eab5bc1. Report an issue: GitHub.