macOS: Clear NSOpenGLContex's drawable when using offscreen surfaces

Otherwise the user might accidentally render to the previously active
window, if not explicitly using an FBO.

This will have an performance impact if doing makeCurrent on a real
window and an offscreen window back and forth with the same context,
but that's not really a common or recommended use of QOffscreenSurface,
as you can create FBOs with a normal window current as well. The use
case of QOffscreenSurface is when a real window is not available.

Change-Id: If93d04f82564523e15d5970429afea34c5cd31fe
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 29 insertions, 21 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoaglcontext.mm b/src/plugins/platforms/cocoa/qcocoaglcontext.mm
index 2c8885e419..cf4ecd335c 100644
--- a/src/plugins/platforms/cocoa/qcocoaglcontext.mm
+++ b/src/plugins/platforms/cocoa/qcocoaglcontext.mm
@@ -331,34 +331,32 @@ bool QCocoaGLContext::makeCurrent(QPlatformSurface *surface)
 
     Q_ASSERT(surface->surface()->supportsOpenGL());
 
-    if (surface->surface()->surfaceClass() == QSurface::Offscreen) {
-        [m_context makeCurrentContext];
-        return true;
-    }
-
     if (!setDrawable(surface))
         return false;
 
     [m_context makeCurrentContext];
 
-    // Disable high-resolution surfaces when using the software renderer, which has the
-    // problem that the system silently falls back to a to using a low-resolution buffer
-    // when a high-resolution buffer is requested. This is not detectable using the NSWindow
-    // convertSizeToBacking and backingScaleFactor APIs. A typical result of this is that Qt
-    // will display a quarter of the window content when running in a virtual machine.
-    if (!m_didCheckForSoftwareContext) {
-        // FIXME: This ensures we check only once per context,
-        // but the context may be used for multiple surfaces.
-        m_didCheckForSoftwareContext = true;
-
-        const GLubyte* renderer = glGetString(GL_RENDERER);
-        if (qstrcmp((const char *)renderer, "Apple Software Renderer") == 0) {
-            NSView *view = static_cast<QCocoaWindow *>(surface)->m_view;
-            [view setWantsBestResolutionOpenGLSurface:NO];
+    if (surface->surface()->surfaceClass() == QSurface::Window) {
+        // Disable high-resolution surfaces when using the software renderer, which has the
+        // problem that the system silently falls back to a to using a low-resolution buffer
+        // when a high-resolution buffer is requested. This is not detectable using the NSWindow
+        // convertSizeToBacking and backingScaleFactor APIs. A typical result of this is that Qt
+        // will display a quarter of the window content when running in a virtual machine.
+        if (!m_didCheckForSoftwareContext) {
+            // FIXME: This ensures we check only once per context,
+            // but the context may be used for multiple surfaces.
+            m_didCheckForSoftwareContext = true;
+
+            const GLubyte* renderer = glGetString(GL_RENDERER);
+            if (qstrcmp((const char *)renderer, "Apple Software Renderer") == 0) {
+                NSView *view = static_cast<QCocoaWindow *>(surface)->m_view;
+                [view setWantsBestResolutionOpenGLSurface:NO];
+            }
         }
+
+        update();
     }
 
-    update();
     return true;
 }
 
@@ -368,7 +366,17 @@ bool QCocoaGLContext::makeCurrent(QPlatformSurface *surface)
 */
 bool QCocoaGLContext::setDrawable(QPlatformSurface *surface)
 {
-    Q_ASSERT(surface && surface->surface()->surfaceClass() == QSurface::Window);
+    if (!surface || surface->surface()->surfaceClass() == QSurface::Offscreen) {
+        // Clear the current drawable and reset the active window, so that GL
+        // commands that don't target a specific FBO will not end up stomping
+        // on the previously set drawable.
+        qCDebug(lcQpaOpenGLContext) << "Clearing current drawable" << m_context.view << "for" << m_context;
+        [m_context clearDrawable];
+        m_currentWindow.clear();
+        return true;
+    }
+
+    Q_ASSERT(surface->surface()->surfaceClass() == QSurface::Window);
     QWindow *window = static_cast<QCocoaWindow *>(surface)->window();
 
     if (window == m_currentWindow.data())