Try (and fail) to avoid a qApp->processEvents() call

I just can't find any other reliable way to wait until javascript
execution in a web page has completed.

python/core/layout/qgslayoutitemhtml.sip

@@ -261,6 +261,7 @@ Recalculates the frame sizes for the current viewport dimensions
 
 };
 
+
 /************************************************************************
  * This file has been generated automatically from                      *
  *                                                                      *

src/core/layout/qgslayoutitemhtml.cpp

@@ -197,9 +197,12 @@ void QgsLayoutItemHtml::loadHtml( const bool useCache, const QgsExpressionContex
   //inject JSON feature
   if ( !mAtlasFeatureJSON.isEmpty() )
   {
-    mWebPage->mainFrame()->evaluateJavaScript( QStringLiteral( "if ( typeof setFeature === \"function\" ) { setFeature(%1); }" ).arg( mAtlasFeatureJSON ) );
-    //needs an extra process events here to give JavaScript a chance to execute
-    qApp->processEvents();
+    JavascriptExecutorLoop jsLoop;
+
+    mWebPage->mainFrame()->addToJavaScriptWindowObject( "loop", &jsLoop );
+    mWebPage->mainFrame()->evaluateJavaScript( QStringLiteral( "if ( typeof setFeature === \"function\" ) { setFeature(%1); }; loop.done();" ).arg( mAtlasFeatureJSON ) );
+
+    jsLoop.execIfNotDone();
   }
 
   recalculateFrameSizes();
@@ -533,3 +536,25 @@ void QgsLayoutItemHtml::refreshDataDefinedProperty( const QgsLayoutObject::DataD
     loadHtml( true, &context );
   }
 }
+
+//JavascriptExecutorLoop
+///@cond PRIVATE
+
+void JavascriptExecutorLoop::done()
+{
+  mDone = true;
+  quit();
+}
+
+void JavascriptExecutorLoop::execIfNotDone()
+{
+  if ( !mDone )
+    exec( QEventLoop::ExcludeUserInputEvents );
+
+  // gross, but nothing else works, so f*** it.. it's not worth spending a day trying to find a non-hacky way
+  // to force the web page to update following the js execution
+  for ( int i = 0; i < 100; i++ )
+    qApp->processEvents();
+}
+
+///@endcond

src/core/layout/qgslayoutitemhtml.h

@@ -276,4 +276,22 @@ class CORE_EXPORT QgsLayoutItemHtml: public QgsLayoutMultiFrame
     void refreshExpressionContext();
 };
 
+///@cond PRIVATE
+#ifndef SIP_RUN
+class JavascriptExecutorLoop : public QEventLoop
+{
+    Q_OBJECT
+  public slots:
+
+    void done();
+    void execIfNotDone();
+
+  private:
+
+    bool mDone = false;
+
+};
+#endif
+///@endcond
+
 #endif // QGSLAYOUTITEMHTML_H