Prevent data loss when closing LyX while document dialog has unapplied changes (#12955)

Entails new strings, but I think the severity allows for that.

src/frontends/qt/Dialog.h

@@ -267,6 +267,8 @@ protected:
 	/// To be called when the buffer view has changed
 	virtual void onBufferViewChanged() = 0;
 	///
+	virtual void onClosing(int) = 0;
+	///
 	void connectToNewInset();
 
 private:

src/frontends/qt/DialogView.cpp

@@ -25,6 +25,8 @@ DialogView::DialogView(GuiView & lv, QString const & name, QString const & title
 {
 	connect(&lv, SIGNAL(bufferViewChanged()),
 	        this, SLOT(onBufferViewChanged()));
+	connect(&lv, SIGNAL(closing(int)),
+		this, SLOT(onClosing(int)));
 
 	// remove question marks from Windows dialogs
 	setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);

src/frontends/qt/DialogView.h

@@ -48,6 +48,7 @@ protected:
 
 protected Q_SLOTS:
 	void onBufferViewChanged() override {}
+	void onClosing(int) override {}
 };
 
 } // namespace frontend

src/frontends/qt/DockView.h

@@ -56,6 +56,7 @@ public:
 
 protected Q_SLOTS:
 	void onBufferViewChanged() override {}
+	void onClosing(int) override {}
 };
 
 } // namespace frontend

src/frontends/qt/GuiDialog.cpp

@@ -34,6 +34,8 @@ GuiDialog::GuiDialog(GuiView & lv, QString const & name, QString const & title)
 {
 	connect(&lv, SIGNAL(bufferViewChanged()),
 	        this, SLOT(onBufferViewChanged()));
+	connect(&lv, SIGNAL(closing(int)),
+		this, SLOT(onClosing(int)));
 
 	// remove question marks from Windows dialogs
 	setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);

src/frontends/qt/GuiDialog.h

@@ -66,6 +66,7 @@ public Q_SLOTS:
 
 protected Q_SLOTS:
 	void onBufferViewChanged() override {}
+	void onClosing(int) override {}
 
 public:
 	/** Check whether we may apply our data.

src/frontends/qt/GuiDocument.cpp

@@ -1849,6 +1849,23 @@ GuiDocument::GuiDocument(GuiView & lv)
 }
 
 
+void GuiDocument::onClosing(int const id)
+{
+	if (!guiApp || !guiApp->currentView()
+	    || guiApp->currentView()->id() != id
+	    || !bc().policy().buttonStatus(ButtonPolicy::RESTORE))
+		// notthing to do
+		return;
+
+	int const ret = Alert::prompt(_("Unapplied changes"),
+			_("Some changes in the document were not yet applied.\n"
+			"Do you want to apply them before closing?"),
+			1, 1, _("Yes, &Apply"), _("No, &Dismiss Changes"));
+	if (ret == 0)
+		slotOK();
+}
+
+
 void GuiDocument::onBufferViewChanged()
 {
 	if (switchback_) {
@@ -1868,8 +1885,8 @@ void GuiDocument::onBufferViewChanged()
 		// Only ask if we haven't yet in this cycle
 		int const ret = prompted_ ? 3 : Alert::prompt(_("Unapplied changes"),
 				_("Some changes in the previous document were not yet applied.\n"
-				"Do you want to switch back and apply them?"),
-				1, 1, _("Yes, &Switch Back"), _("No, &Dismiss Changes"));
+				"Do you want to switch back in order to apply them or dismiss the changes?"),
+				1, 1, _("&Switch Back"), _("&Dismiss Changes"));
 		if (ret == 0) {
 			// Switch to previous buffer view and apply
 			switchback_ = true;

src/frontends/qt/GuiDocument.h

@@ -96,6 +96,7 @@ public:
 
 public Q_SLOTS:
 	void onBufferViewChanged() override;
+	void onClosing(int) override;
 	// OK button clicked
 	void slotOK();
 	// Apply button clicked

src/frontends/qt/GuiView.cpp

@@ -1311,6 +1311,7 @@ void GuiView::closeEvent(QCloseEvent * close_event)
 	// it can happen that this event arrives without selecting the view,
 	// e.g. when clicking the close button on a background window.
 	setFocus();
+	Q_EMIT closing(id_);
 	if (!closeWorkAreaAll()) {
 		closing_ = false;
 		close_event->ignore();