Added check before redraw in ContourPreviewPlot Re #28436

Without these added checks the contour plot was being redrawn any time any workspace was replaced or deleted from the ADS leading to significant slowdowns in some operations when the interface was open.

qt/widgets/plotting/src/Mpl/ContourPreviewPlot.cpp

@@ -68,11 +68,15 @@ void ContourPreviewPlot::onWorkspaceRemoved(
   if (auto workspace =
           boost::dynamic_pointer_cast<MatrixWorkspace>(nf->object())) {
     // If the artist has already been removed, ignore.
+    bool workspaceRemoved = false;
     try {
-      m_canvas->gca<MantidAxes>().removeWorkspaceArtists(workspace);
+      workspaceRemoved =
+          m_canvas->gca<MantidAxes>().removeWorkspaceArtists(workspace);
     } catch (Mantid::PythonInterface::PythonException &) {
     }
-    m_canvas->draw();
+    if (workspaceRemoved) {
+      m_canvas->draw();
+    }
   }
 }
 
@@ -86,8 +90,9 @@ void ContourPreviewPlot::onWorkspaceReplaced(
           boost::dynamic_pointer_cast<MatrixWorkspace>(nf->oldObject())) {
     if (auto newWorkspace =
             boost::dynamic_pointer_cast<MatrixWorkspace>(nf->newObject())) {
-      m_canvas->gca<MantidAxes>().replaceWorkspaceArtists(newWorkspace);
-      m_canvas->draw();
+      if (m_canvas->gca<MantidAxes>().replaceWorkspaceArtists(newWorkspace)) {
+        m_canvas->draw();
+      }
     }
   }
 }