Further ensure scriptlets are actually injected

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/ye6abt/

Possibly because the Opera sidebar window is a special
case, it appears the scriptlets must be injected at a
later time.

Use a global isolated window variable to detect whether
the scriptlets have really be injected, and ultimately
inject them at main content script time when it is found
they haven't been injected at that point.

src/js/contentscript.js

@@ -1331,7 +1331,8 @@ vAPI.DOMFilterer = class {
 
         // Library of resources is located at:
         // https://github.com/gorhill/uBlock/blob/master/assets/ublock/resources.txt
-        if ( scriptlets ) {
+        if ( scriptlets && typeof self.uBO_scriptletsInjected !== 'boolean' ) {
+            self.uBO_scriptletsInjected = true;
             vAPI.injectScriptlet(document, scriptlets);
             vAPI.injectedScripts = scriptlets;
         }
@@ -1362,9 +1363,7 @@ vAPI.DOMFilterer = class {
         vAPI.messaging.send('contentscript', {
             what: 'retrieveContentScriptParameters',
             url: vAPI.effectiveSelf.location.href,
-            scriptletsInjected:
-                typeof self.uBO_scriptletsInjected === 'boolean' &&
-                self.uBO_scriptletsInjected,
+            needScriptlets: typeof self.uBO_scriptletsInjected !== 'boolean',
         }).then(response => {
             bootstrapPhase1(response);
         });

src/js/messaging.js

@@ -773,8 +773,8 @@ const retrieveContentScriptParameters = async function(sender, request) {
     // https://github.com/uBlockOrigin/uBlock-issues/issues/688#issuecomment-748179731
     //   For non-network URIs, scriptlet injection is deferred to here. The
     //   effective URL is available here in `request.url`.
-    if ( request.scriptletsInjected !== true ) {
-        scriptletFilteringEngine.injectNow(request);
+    if ( request.needScriptlets ) {
+        response.scriptlets = scriptletFilteringEngine.injectNow(request);
     }
 
     // https://github.com/NanoMeow/QuickReports/issues/6#issuecomment-414516623

src/js/scriptlet-filtering.js

@@ -432,13 +432,17 @@ scriptletFilteringEngine.injectNow = function(details) {
         matchAboutBlank: true,
         runAt: 'document_start',
     });
-    if ( logEntries === undefined ) { return; }
-    promise.then(results => {
-        if ( Array.isArray(results) === false || results[0] !== 0 ) { return; }
-        for ( const entry of logEntries ) {
-            logOne(entry.tabId, entry.url, entry.token);
-        }
-    });
+    if ( logEntries !== undefined ) {
+        promise.then(results => {
+            if ( Array.isArray(results) === false || results[0] !== 0 ) {
+                return;
+            }
+            for ( const entry of logEntries ) {
+                logOne(entry.tabId, entry.url, entry.token);
+            }
+        });
+    }
+    return scriptlets;
 };
 
 scriptletFilteringEngine.toSelfie = function() {