fix multiselect combobox

go/wasmruntime/host_wasm.go

@@ -208,6 +208,30 @@ func SetScrollLeft(r *vdom.Ref, x float64) {
 	}
 }
 
+// OverflowsX reports whether an element's content is wider than the box it is
+// clipped to — i.e. something is hidden.
+//
+// It compares scrollWidth (the full content) against clientWidth (the visible content
+// box). Measure/getBoundingClientRect is the WRONG comparison here: it reports the
+// element's own border box, which is by definition the size it was clipped to, so it
+// can never reveal an overflow.
+//
+// A detached or display:none element has no layout and reports 0/0; that is not an
+// overflow, so it answers false rather than a misleading true.
+func OverflowsX(r *vdom.Ref) bool {
+	n, ok := node(r)
+	if !ok {
+		return false
+	}
+	client := n.Get("clientWidth").Float()
+	if client == 0 {
+		return false
+	}
+	// Sub-pixel layout means scrollWidth can exceed clientWidth by a hair on content
+	// that visually fits. Round up to whole pixels before believing it.
+	return n.Get("scrollWidth").Float() > client+1
+}
+
 // ---- hit testing (outside-click) ----
 
 // Contains reports whether target lies inside r's subtree. target is an