Avoid 'flashy' status bar progress widget when map renders are fast

If previous canvas render took less than 0.5 seconds, delay the appearance of the
'render in progress' status bar widget by a short amount - this avoids the
status bar rapidly appearing and then disappearing for very fast renders

Author: nyalldawson

src/app/qgisapp.cpp

@@ -8282,11 +8282,32 @@ void QgisApp::refreshMapCanvas()
 
 void QgisApp::canvasRefreshStarted()
 {
-  showProgress( -1, 0 ); // trick to make progress bar show busy indicator
+  mLastRenderTime.restart();
+  // if previous render took less than 0.5 seconds, delay the appearance of the
+  // render in progress status bar by 0.5 seconds - this avoids the status bar
+  // rapidly appearing and then disappearing for very fast renders
+  if ( mLastRenderTimeSeconds > 0 && mLastRenderTimeSeconds < 0.5 )
+  {
+    mRenderProgressBarTimer.setSingleShot( true );
+    mRenderProgressBarTimer.setInterval( 500 );
+    disconnect( mRenderProgressBarTimerConnection );
+    mRenderProgressBarTimerConnection = connect( &mRenderProgressBarTimer, &QTimer::timeout, [ = ]()
+    {
+      showProgress( -1, 0 );
+    }
+                                               );
+    mRenderProgressBarTimer.start();
+  }
+  else
+  {
+    showProgress( -1, 0 ); // trick to make progress bar show busy indicator
+  }
 }
 
 void QgisApp::canvasRefreshFinished()
 {
+  mRenderProgressBarTimer.stop();
+  mLastRenderTimeSeconds = mLastRenderTime.elapsed() / 1000.0;
   showProgress( 0, 0 ); // stop the busy indicator
 }
 

src/app/qgisapp.h

@@ -2067,6 +2067,11 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
 
     QgsStatusBar *mStatusBar = nullptr;
 
+    QTime mLastRenderTime;
+    double mLastRenderTimeSeconds = 0;
+    QTimer mRenderProgressBarTimer;
+    QMetaObject::Connection mRenderProgressBarTimerConnection;
+
     friend class TestQgisAppPython;
 };