fix multiselect combobox

go/wasmruntime/testdata/domexec.js

@@ -42,6 +42,11 @@ class DNode {
 		// Tests set .rect to control what getBoundingClientRect reports; there is no
 		// layout engine here, so geometry is whatever the test declares.
 		this.rect = { left: 0, top: 0, width: 0, height: 0 };
+		// Likewise for overflow. clientWidth is the visible content box, scrollWidth the
+		// full content — content overflows exactly when the second exceeds the first, and
+		// a test declares both rather than a layout engine deriving them.
+		this.clientWidth = 0;
+		this.scrollWidth = 0;
 	}
 	get nodeType() { return this.tag === "#text" ? 3 : 1; }
 	get firstChild() { return this.childNodes[0] ?? null; }