Gui: Reset scroll position after preference page change

As every page is contained in the same QScrollArea we must ensure that
after page is changed the scrolled area is scrolled back to top-left
corner.

Fixes: #11981

src/Gui/DlgPreferencesImp.cpp

@@ -178,7 +178,7 @@ void DlgPreferencesImp::setupPages()
         }
     }
 
-    updatePageDependentLabels();
+    updatePageDependentWidgets();
 }
 
 QPixmap DlgPreferencesImp::loadIconForGroup(const std::string &name) const
@@ -317,11 +317,16 @@ void DlgPreferencesImp::createPageInGroup(PreferencesPageItem *groupItem, const
     }
 }
 
-void DlgPreferencesImp::updatePageDependentLabels()
+void DlgPreferencesImp::updatePageDependentWidgets()
 {
     auto currentPageItem = getCurrentPage();
 
+    // update header of the page
     ui->headerLabel->setText(currentPageItem->text());
+
+    // reset scroll area to start position
+    ui->scrollArea->horizontalScrollBar()->setValue(0);
+    ui->scrollArea->verticalScrollBar()->setValue(0);
 }
 
 /**
@@ -430,7 +435,7 @@ void DlgPreferencesImp::activateGroupPage(const QString& group, int index)
             ui->groupWidgetStack->setCurrentWidget(pageStackWidget);
             pageStackWidget->setCurrentIndex(index);
 
-            updatePageDependentLabels();
+            updatePageDependentWidgets();
             
             return;
         }
@@ -745,7 +750,7 @@ void DlgPreferencesImp::onPageSelected(const QModelIndex& index)
         pagesStackWidget->setCurrentIndex(index.row());
     }
 
-    updatePageDependentLabels();
+    updatePageDependentWidgets();
 }
 
 void DlgPreferencesImp::onGroupExpanded(const QModelIndex& index)
@@ -824,7 +829,7 @@ void DlgPreferencesImp::changeEvent(QEvent *e)
             }
         }
 
-        updatePageDependentLabels();
+        updatePageDependentWidgets();
     } else {
         QWidget::changeEvent(e);
     }

src/Gui/DlgPreferencesImp.h

@@ -178,7 +178,7 @@ private:
     void restorePageDefaults(PreferencesPageItem* item);
     void restartIfRequired();
 
-    void updatePageDependentLabels();
+    void updatePageDependentWidgets();
 
     QPixmap loadIconForGroup(const std::string& name) const;
     //@}