Clipboard: Retry on_dataChanged() after a delay on windows (#10109)

An undocumented behaviour of QClipboard::mimeData() is that it can fail on
windows due to the specificities of the windows API that allow a race condition.
In particular it seems that querying the clipboard as soon as the dataChanged()
signal is received favourises this race condition.

Thanks to Trac user bquistorff for the explanation and a proof of concept patch.

src/frontends/qt4/GuiClipboard.cpp

@@ -112,7 +112,7 @@ GuiClipboard::GuiClipboard()
 	connect(qApp->clipboard(), SIGNAL(dataChanged()),
 		this, SLOT(on_dataChanged()));
 	// initialize clipboard status.
-	on_dataChanged();
+	update();
 }
 
 
@@ -550,6 +550,17 @@ bool GuiClipboard::hasInternal() const
 
 
 void GuiClipboard::on_dataChanged()
+{
+	update();
+#if defined(Q_OS_WIN) || defined(Q_CYGWIN_WIN)
+	// Retry on Windows (#10109)
+	if (cache_.formats().count() == 0) {
+		QTimer::singleShot(100, this, SLOT(update()));
+	}
+#endif
+}
+
+void GuiClipboard::update()
 {
 	//Note: we do not really need to run cache_.update() unless the
 	//data has been changed *and* the GuiClipboard has been queried.

src/frontends/qt4/GuiClipboard.h

@@ -84,6 +84,7 @@ public:
 
 private Q_SLOTS:
 	void on_dataChanged();
+	void update();
 
 private:
 	bool plaintext_clipboard_empty_;