tdf#119720 KDE5: Don't show hidden windows on modality change

This change fixes clipping and drawing issues
for document recovery window and other modal windows.

Change-Id: I005254d4cd2598f4f2d02ba7e7bbf7388affe36c
Reviewed-on: https://gerrit.libreoffice.org/68307
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

vcl/qt5/Qt5Frame.cxx

@@ -493,13 +493,21 @@ void Qt5Frame::SetModal(bool bModal)
 {
     if (isWindow())
     {
-        if (m_pTopLevel)
-            m_pTopLevel->setVisible(true);
+        bool wasVisible = windowHandle()->isVisible();
+
         // modality change is only effective if the window is hidden
-        windowHandle()->hide();
+        if (wasVisible)
+        {
+            windowHandle()->hide();
+        }
+
         windowHandle()->setModality(bModal ? Qt::WindowModal : Qt::NonModal);
-        // and shown again
-        windowHandle()->show();
+
+        // and shown again if it was visible
+        if (wasVisible)
+        {
+            windowHandle()->show();
+        }
     }
 }