Updated Blazor macOS Catalina/iOS 13 workaround (#14576)

* MacOSX Catalina Beta wasm workaround for booting mono wasm from Blazor.

* Address review comments

* Address review comments

* Make it clear why global scope is used.

* Stylistic tweaks for Catalina workaround

* Further streamlining

* Update js binaries

src/Components/Web.JS/src/Platform/Mono/MonoPlatform.ts

@@ -25,10 +25,14 @@ export const monoPlatform: Platform = {
       window['Browser'] = {
         init: () => { },
       };
-      // Emscripten works by expecting the module config to be a global
-      window['Module'] = createEmscriptenModuleInstance(loadAssemblyUrls, resolve, reject);
 
-      addScriptTagsToDocument();
+      // Emscripten works by expecting the module config to be a global
+      // For compatibility with macOS Catalina, we have to assign a temporary value to window.Module
+      // before we start loading the WebAssembly files
+      addGlobalModuleScriptTagsToDocument(() => {
+        window['Module'] = createEmscriptenModuleInstance(loadAssemblyUrls, resolve, reject);
+        addScriptTagsToDocument();
+      });
     });
   },
 
@@ -205,6 +209,23 @@ function addScriptTagsToDocument() {
   document.body.appendChild(scriptElem);
 }
 
+// Due to a strange behavior in macOS Catalina, we have to delay loading the WebAssembly files
+// until after it finishes evaluating a <script> element that assigns a value to window.Module.
+// This may be fixed in a later version of macOS/iOS, or even if not it may be possible to reduce
+// this to a smaller workaround.
+function addGlobalModuleScriptTagsToDocument(callback: () => void) {
+  const scriptElem = document.createElement('script');
+
+  // This pollutes global but is needed so it can be called from the script.
+  // The callback is put in the global scope so that it can be run after the script is loaded.
+  // onload cannot be used in this case for non-file scripts.
+  window['__wasmmodulecallback__'] = callback;
+  scriptElem.type = 'text/javascript';
+  scriptElem.text = 'var Module; window.__wasmmodulecallback__(); delete window.__wasmmodulecallback__;';
+
+  document.body.appendChild(scriptElem);
+}
+
 function createEmscriptenModuleInstance(loadAssemblyUrls: string[], onReady: () => void, onError: (reason?: any) => void) {
   const module = {} as typeof Module;
   const wasmBinaryFile = '_framework/wasm/mono.wasm';