zed-industries/zed · error

creating text OffscreenCanvas

Error message

creating text OffscreenCanvas: {error:?}

What it means

TextCanvas::new constructs a browser OffscreenCanvas via web_sys. If the JS constructor throws (the JsValue error is captured with {error:?}), it is wrapped in this anyhow error. This typically means the browser/runtime does not support OffscreenCanvas or the width/height arguments were rejected.

Solutions

1. Use a browser that supports OffscreenCanvas (Chrome 69+, Firefox 105+, Safari 16.4+).
2. Guard with a feature check: `if js_sys::Reflect::has(&window, &"OffscreenCanvas".into())` before constructing, and fall back to a DOM canvas.
3. Verify width/height passed to TextCanvas::new are non-negative integers.

Example fix

// before
let canvas = TextCanvas::new(width, height)?;
// after
if !has_offscreen_canvas_support() {
    return Err(anyhow!("OffscreenCanvas unsupported; use DOM canvas fallback"));
}
let canvas = TextCanvas::new(width, height)?;

function supportsOffscreenCanvas() {
  return typeof OffscreenCanvas !== "undefined";
}

fn has_offscreen_canvas(window: &web_sys::Window) -> bool {
    js_sys::Reflect::has(&window, &js_sys::Symbol::from("OffscreenCanvas"))
        .unwrap_or(false)
}

match TextCanvas::new(width, height) {
    Ok(canvas) => canvas,
    Err(e) => {
        log::warn!("OffscreenCanvas unavailable: {e:#}; falling back to DOM canvas");
        TextCanvas::from_dom_canvas(width, height)?
    }
};

When it happens

Trigger: Calling TextCanvas::new in a browser without OffscreenCanvas support (e.g. older Safari, non-secure contexts in some engines) or with dimensions the engine refuses.

Common situations: Running a Zed web build in an outdated browser or a JS environment (older WebView, worker without OffscreenCanvas) where `new OffscreenCanvas` is undefined/throws.

Understand the failure class

Background: "unsupported platform" / "not supported on this platform" errors: what they mean and how to fix them — this error's family across 47 libraries.

Related errors

• getting OffscreenCanvas 2D context
• Canvas text raster returned
• setting Canvas readback option
• unexpected OffscreenCanvas 2D context
• Canvas readback option could not be set

AI-assisted analysis of zed-industries/zed@916fc2b8cb (2026-09-19). Data as JSON: /api/errors/211f5a7ea518929a. Report an issue: GitHub.

Appendix: source

Thrown at crates/gpui_web/src/canvas_text.rs:31

#[derive(Clone, Copy, Debug)]
pub(crate) struct CanvasTextMetrics {
    pub(crate) advance: f32,
    pub(crate) left: f32,
    pub(crate) right: f32,
    pub(crate) ascent: f32,
    pub(crate) descent: f32,
}

struct TextCanvas {
    canvas: OffscreenCanvas,
    context: OffscreenCanvasRenderingContext2d,
}

impl TextCanvas {
    fn new() -> Result<Self> {
        let canvas = OffscreenCanvas::new(1, 1)
            .map_err(|error| anyhow!("creating text OffscreenCanvas: {error:?}"))?;
        let options = js_sys::Object::new();
        // Every rasterized glyph is read back for upload into GPUI's atlas.
        let assigned = js_sys::Reflect::set(
            &options,
            &JsValue::from_str("willReadFrequently"),
            &JsValue::TRUE,
        )
        .map_err(|error| anyhow!("setting Canvas readback option: {error:?}"))?;
        ensure!(assigned, "Canvas readback option could not be set");
        let context = canvas
            .get_context_with_context_options("2d", &options)
            .map_err(|error| anyhow!("getting OffscreenCanvas 2D context: {error:?}"))?
            .context("OffscreenCanvas 2D text rendering is unavailable")?
            .dyn_into::<OffscreenCanvasRenderingContext2d>()
            .map_err(|error| anyhow!("unexpected OffscreenCanvas 2D context: {error:?}"))?;
        Ok(Self { canvas, context })
    }

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