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Await page.WaitForLoadStateAsync (DOMContentLoaded/NetworkIdle) before humanize calls

CloakHQ/CloakBrowser · error · StealthEvaluationError

<viewport>

Error message

<viewport>

What it means

StealthEvaluationError thrown by GetLiveWindowSizeAsync when evaluating the stealth viewport JS in the isolated world fails to return an object with width/height properties — i.e. the probe result is null, not JSON, or malformed.

Source

Thrown at dotnet/src/CloakBrowser/Human/PlaywrightAdapters.cs:102

    }

    public (int Width, int Height)? ViewportSize
    {
        get
        {
            var vs = _page.ViewportSize;
            return vs == null ? null : (vs.Width, vs.Height);
        }
    }

    public async Task<(int Width, int Height)?> GetLiveWindowSizeAsync()
    {
        var world = await _getStealthAsync().ConfigureAwait(false);
        var value = await world.EvaluateAsync(StealthDom.ViewportJs).ConfigureAwait(false);
        if (value == null || value.Value.ValueKind != System.Text.Json.JsonValueKind.Object
            || !value.Value.TryGetProperty("width", out var width)
            || !value.Value.TryGetProperty("height", out var height))
            throw new StealthEvaluationError("<viewport>");

        int resolvedWidth = (int)width.GetDouble();
        int resolvedHeight = (int)height.GetDouble();
        if (resolvedWidth <= 0 || resolvedHeight <= 0)
            throw new StealthEvaluationError("<viewport>");
        return (resolvedWidth, resolvedHeight);
    }

    public async Task<(double Y, double MaxY)?> GetScrollStateAsync()
    {
        var world = await _getStealthAsync().ConfigureAwait(false);
        var value = await world.EvaluateAsync(
            "(() => { const e = document.scrollingElement || document.documentElement;" +
            " return { y: window.scrollY, maxY: Math.max(0, e.scrollHeight - e.clientHeight) }; })()")
            .ConfigureAwait(false);
        if (value == null || value.Value.ValueKind != System.Text.Json.JsonValueKind.Object
            || !value.Value.TryGetProperty("y", out var y)
            || !value.Value.TryGetProperty("maxY", out var maxY))

View on GitHub (pinned to d6bad5de26)

Solutions

Await page.WaitForLoadStateAsync (DOMContentLoaded/NetworkIdle) before humanize calls
Check page.IsClosed and that the stealth world is available before calling
Let CloakBrowser create the context so stealth scripts are guaranteed installed
Retry once after a short delay if a navigation race is suspected

Example fix

// before
await page.GoToAsync(url);
await page.HumanScrollIntoViewAsync(sel);

// after
await page.GoToAsync(url);
await page.WaitForLoadStateAsync(LoadState.DOMContentLoaded);
await page.HumanScrollIntoViewAsync(sel);

Defensive patterns

Strategy: retry

Validate before calling

await page.WaitForLoadStateAsync(LoadState.DOMContentLoaded);
if (page.IsClosed) return;

Try / catch

catch (StealthEvaluationError e) when (e.Message == "<viewport>") { await Task.Delay(250); retry once; }

Prevention

Set explicit viewport to avoid the live-dimension fallback path
Never call humanize APIs mid-navigation
Create contexts via CloakBrowser so the stealth world exists

When it happens

Trigger: Calling APIs that need live window size (scroll-into-view fallback in headed no_viewport mode) when the stealth world's EvaluateAsync returns null or a non-object — page mid-navigation, page crashed, or stealth scripts not injected.

Common situations: Calling humanize APIs right after GoToAsync before load completes, on a crashed page, or when the isolated world/stealth scripts were not installed (custom context setup bypassing CloakBrowser defaults).

Related errors

AI-assisted analysis of CloakHQ/CloakBrowser@d6bad5de26 (2026-08-28). Data as JSON: /api/errors/1165ef7bf1c19842. Report an issue: GitHub.