Add dialog box if necessary + fix checkbox status

interfaces/akregator.kcfg

@@ -10,6 +10,13 @@
       <whatsthis>Hides feeds with no unread articles</whatsthis>
       <default>false</default>
   </entry>
+  <entry key="ShowMenuBar" type="Bool">
+    <default>true</default>
+     <!-- label and whatsthis are already provided by KStandardAction::showMenubar -->
+     <label></label>
+     <whatsthis></whatsthis>
+  </entry>
+
   <entry key="Lock Feeds In Place" type="Bool">
       <label>Prevent feeds from being moved in the list</label>
       <whatsthis>Locks feeds where they are</whatsthis>

src/mainwindow.cpp

@@ -58,7 +58,9 @@ MainWindow::MainWindow(QWidget *parent)
     statusBar()->addWidget(m_statusLabel, 1);
 
     KStandardAction::quit(qApp, &QApplication::quit, actionCollection());
-    mShowMenuBarAction = KStandardAction::showMenubar(menuBar(), &QMenuBar::setVisible, actionCollection());
+    mShowMenuBarAction = KStandardAction::showMenubar(this, &MainWindow::slotToggleMenubar, actionCollection());
+    mShowMenuBarAction->setChecked(Settings::showMenuBar());
+    slotToggleMenubar(true);
 
     if (menuBar()) {
         mHamburgerMenu = KStandardAction::hamburgerMenu(nullptr, nullptr, actionCollection());
@@ -81,6 +83,27 @@ MainWindow::MainWindow(QWidget *parent)
     connect(qApp, &QCoreApplication::aboutToQuit, this, &MainWindow::slotOnShutdown);
 }
 
+void MainWindow::slotToggleMenubar(bool dontShowWarning)
+{
+    if (menuBar()) {
+        if (mShowMenuBarAction->isChecked()) {
+            menuBar()->show();
+        } else {
+            if (!dontShowWarning && (!toolBar()->isVisible() || !toolBar()->actions().contains(mHamburgerMenu))) {
+                const QString accel = mShowMenuBarAction->shortcut().toString();
+                KMessageBox::information(this,
+                                         i18n("<qt>This will hide the menu bar completely."
+                                              " You can show it again by typing %1.</qt>",
+                                              accel),
+                                         i18n("Hide menu bar"),
+                                         QStringLiteral("HideMenuBarWarning"));
+            }
+            menuBar()->hide();
+        }
+        Settings::setShowMenuBar(mShowMenuBarAction->isChecked());
+    }
+}
+
 void MainWindow::updateHamburgerMenu()
 {
     QMenu *menu = new QMenu(this);

src/mainwindow.h

@@ -81,6 +81,7 @@ private Q_SLOTS:
 
 private:
     void updateHamburgerMenu();
+    void slotToggleMenubar(bool dontShowWarning);
     QPointer<Akregator::Part> m_part;
     KSqueezedTextLabel *const m_statusLabel;
     KHamburgerMenu *mHamburgerMenu = nullptr;