Clear QCocoaGLContext's m_currentWindow when window is hidden.

Task-number: QTBUG-35363
Change-Id: I1b3d883ed10200af8a2d4188fb1725b36eb78022
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

src/plugins/platforms/cocoa/qcocoaglcontext.h

@@ -75,6 +75,8 @@ public:
     bool isSharing() const;
     bool isValid() const;
 
+    void windowWasHidden();
+
 private:
     void setActiveWindow(QWindow *window);
     void updateSurfaceFormat();

src/plugins/platforms/cocoa/qcocoaglcontext.mm

@@ -165,6 +165,16 @@ QSurfaceFormat QCocoaGLContext::format() const
     return m_format;
 }
 
+void QCocoaGLContext::windowWasHidden()
+{
+    // If the window is hidden, we need to unset the m_currentWindow
+    // variable so that succeeding makeCurrent's will not abort prematurely
+    // because of the optimization in setActiveWindow.
+    // Doing a full doneCurrent here is not preferable, because the GL context
+    // might be rendering in a different thread at this time.
+    m_currentWindow.clear();
+}
+
 void QCocoaGLContext::swapBuffers(QPlatformSurface *surface)
 {
     QWindow *window = static_cast<QCocoaWindow *>(surface)->window();

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -388,6 +388,8 @@ void QCocoaWindow::setVisible(bool visible)
             [m_contentView setHidden:NO];
     } else {
         // qDebug() << "close" << this;
+        if (m_glContext)
+            m_glContext->windowWasHidden();
         if (m_nsWindow) {
             if (m_hasModalSession) {
                 QCocoaEventDispatcher *cocoaEventDispatcher = qobject_cast<QCocoaEventDispatcher *>(QGuiApplication::instance()->eventDispatcher());