macOS: Fix window state after going out of fullscreen

If you went out of fullscreen with the shortcut or menu item, it would
not restore a previous maximized state, but instead resized the window
to the smaller non-maximized state. So,

1. open Qt Creator and resize to some non-maximized size
2. double-click window title to maximize
3. Window > Enter Full Screen
4. Window > Exit Full Screen

now returns correctly to state 2, not 1.

Change-Id: I076321070d011d73bfe888c0a4a5ade24c1057d6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/windowsupport.cpp

@@ -40,6 +40,7 @@
 #include <QEvent>
 #include <QMenu>
 #include <QWidget>
+#include <QWindowStateChangeEvent>
 
 namespace Core {
 namespace Internal {
@@ -111,6 +112,7 @@ bool WindowSupport::eventFilter(QObject *obj, QEvent *event)
             m_minimizeAction->setEnabled(!minimized);
             m_zoomAction->setEnabled(!minimized);
         }
+        m_previousWindowState = static_cast<QWindowStateChangeEvent *>(event)->oldState();
         updateFullScreenAction();
     } else if (event->type() == QEvent::WindowActivate) {
         m_windowList->setActiveWindow(m_window);
@@ -126,7 +128,7 @@ bool WindowSupport::eventFilter(QObject *obj, QEvent *event)
 void WindowSupport::toggleFullScreen()
 {
     if (m_window->isFullScreen()) {
-        m_window->setWindowState(m_window->windowState() & ~Qt::WindowFullScreen);
+        m_window->setWindowState(m_previousWindowState & ~Qt::WindowFullScreen);
     } else {
         m_window->setWindowState(m_window->windowState() | Qt::WindowFullScreen);
     }

src/plugins/coreplugin/windowsupport.h

@@ -80,6 +80,7 @@ private:
     QAction *m_zoomAction;
     QAction *m_closeAction;
     QAction *m_toggleFullScreenAction;
+    Qt::WindowStates m_previousWindowState;
     bool m_shutdown = false;
 };