{"record":{"id":"3938b609cb852853","repo":"iced-rs/iced","slug":"append-canvas-to-html-body","errorCode":null,"errorMessage":"Append canvas to HTML body","messagePattern":"Append canvas to HTML body","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"winit/src/lib.rs","lineNumber":374,"sourceCode":" let document = window.document().unwrap();\n let body = document.body().unwrap();\n\n let target = target.and_then(|target| {\n body.query_selector(&format!(\"#{target}\"))\n .ok()\n .unwrap_or(None)\n });\n\n match target {\n Some(node) => {\n let _ = node.replace_with_with_node_1(&canvas).expect(\n &format!(\"Could not replace #{}\", node.id()),\n );\n }\n None => {\n let _ = body\n .append_child(&canvas)\n .expect(\"Append canvas to HTML body\");\n }\n };\n }\n\n self.process_event(\n event_loop,\n Event::WindowCreated {\n id,\n window: Arc::new(window),\n exit_on_close_request,\n make_visible: visible,\n on_open,\n },\n );\n }\n Control::Exit => {\n self.process_event(event_loop, Event::Exit);\n event_loop.exit();","sourceCodeStart":356,"sourceCodeEnd":392,"githubUrl":"https://github.com/iced-rs/iced/blob/d146509d893945de5c304cfaf4301335eee278bd/winit/src/lib.rs#L356-L392","documentation":"On wasm32, after obtaining the window canvas the runtime appends it to the document body and expects the DOM append_child call to succeed. append_child returns an Err (HierarchyRequestError or similar) when the body element is missing or the node cannot be attached; iced cannot display anything without the canvas in the DOM, so it panics.","triggerScenarios":"Calling run on wasm when document.body() is None (script executed before exists or in a non-HTML context), or when the canvas node cannot be a child of body (wrong node type/already-attached hierarchy issue).","commonSituations":"WASM module loaded in without defer, so body doesn't exist yet; using a custom loader that runs before the document finishes parsing; page contexts where body was replaced during startup.","solutions":["Load the wasm entry point after the DOM is ready: use \n// after (html)\n","handlingStrategy":"validation","validationCode":"// only mount after the document body exists\nif (document.readyState === \"loading\") {\n await new Promise(r => document.addEventListener(\"DOMContentLoaded\", r));\n}\nif (!document.body) throw new Error(\"document.body missing; cannot mount canvas\");","typeGuard":"function bodyReady(): boolean {\n return document.readyState !== \"loading\" && document.body !== null;\n}","tryCatchPattern":"try {\n body.appendChild(canvas);\n} catch (e) {\n console.error(\"Canvas mount failed; is present and parsed?\", e);\n}","preventionTips":["Load the wasm entry with type=\"module\" or defer so exists.","Never bootstrap the GUI from without deferral.","Confirm the container element id matches the app's query before relying on the append fallback.","Pin compatible iced/winit versions for the web target."],"tags":["wasm","web","dom","winit","panic"],"backgroundTag":"dom-append-failed","analyzedSha":"d146509d893945de5c304cfaf4301335eee278bd","analyzedAt":"2026-09-11T16:54:14.229Z","contentChangedAt":"2026-09-11T16:54:14.229Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}