Terminal block manager: really show the opening message

One pass of the event loop after show() was not enough: the message box
was not painted yet when the long work started (seen on Wayland), only
the busy cursor was visible.

Wait for the window of the message to be exposed by the window system
(500 ms at most), then paint it before building the manager.

sources/TerminalStrip/ui/terminalstripeditorwindow.cpp

@@ -30,7 +30,10 @@
 #include "terminalstriptreedockwidget.h"
 
 #include <QApplication>
+#include <QElapsedTimer>
 #include <QProgressDialog>
+#include <QThread>
+#include <QWindow>
 
 QPointer<TerminalStripEditorWindow> TerminalStripEditorWindow::window_;
 
@@ -53,7 +56,21 @@ TerminalStripEditorWindow *TerminalStripEditorWindow::instance(QETProject *proje
 	busy_.setCancelButton(nullptr);
 	busy_.setMinimumDuration(0);
 	busy_.show();
-		//Paint the message before the long work starts
+		//Paint the message before the long work starts. One pass of the
+		//event loop is not enough : the window is shown by the window system
+		//(Wayland compositor...) a bit later. Wait for it to be exposed,
+		//a short time at most, then paint it.
+	QElapsedTimer timer_;
+	timer_.start();
+	while (timer_.elapsed() < 500)
+	{
+		QCoreApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 20);
+		if (busy_.windowHandle() && busy_.windowHandle()->isExposed()) {
+			break;
+		}
+		QThread::msleep(5);
+	}
+	busy_.repaint();
 	QCoreApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
 	QApplication::setOverrideCursor(Qt::WaitCursor);