macOS: Update NSOpenGLContext drawable on beginning of frame if needed

If the size or DPR of the window we're drawing to with QOpenGLContext
changes we need to call [NSOpenGLContext update] to recreate the backing
texture.

We were doing this in QCocoaGLContext::makeCurrent(), based on the
assumption that clients would always call makeCurrent() before drawing,
which is a documented requirement of QOpenGLContext:

  "To be able to support certain platforms, QOpenGLContext requires
  that you call makeCurrent() again before starting rendering a new
  frame, after calling swapBuffers()."

However RHI doesn't follow this rule, and skips makeCurrent() from
QRhiGles2::ensureContext() if the context is already current.

This was causing issues when moving a window between screens with
different device-pixel-ratios, as we were correctly rendering to
the window with the updated DPR, but we had not resized the target
texture to match.

Changing QRhiGles2::ensureContext() might be an option, but nowadays
we have QPlatformOpenGLContext::beginFrame(), which is guaranteed
to be called (by RHI, not by generic QOpenGLContext clients).

To cater for RHI we now call [NSOpenGLContext update] also from
the beginFrame callback.

Fixes: QTBUG-114873
Pick-to: 6.8 6.5
Change-Id: I9461135109fb1473ef9965f72f52beeb41b03680
Reviewed-by: Morten Johan Sørvig <[email protected]>

2 files changed, 20 insertions, 0 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoaglcontext.h b/src/plugins/platforms/cocoa/qcocoaglcontext.h
index 919f7c240bc..36546879ae4 100644
--- a/src/plugins/platforms/cocoa/qcocoaglcontext.h
+++ b/src/plugins/platforms/cocoa/qcocoaglcontext.h
@@ -29,6 +29,7 @@ public:
     void initialize() override;
 
     bool makeCurrent(QPlatformSurface *surface) override;
+    void beginFrame() override;
     void swapBuffers(QPlatformSurface *surface) override;
     void doneCurrent() override;
 
diff --git a/src/plugins/platforms/cocoa/qcocoaglcontext.mm b/src/plugins/platforms/cocoa/qcocoaglcontext.mm
index a65311175ff..bfac716b633 100644
--- a/src/plugins/platforms/cocoa/qcocoaglcontext.mm
+++ b/src/plugins/platforms/cocoa/qcocoaglcontext.mm
@@ -332,6 +332,25 @@ bool QCocoaGLContext::makeCurrent(QPlatformSurface *surface)
     return true;
 }
 
+void QCocoaGLContext::beginFrame()
+{
+    QMacAutoReleasePool pool;
+
+    Q_ASSERT(context() && context()->surface());
+    auto *surface = context()->surface()->surfaceHandle();
+    Q_ASSERT(surface);
+
+    qCDebug(lcQpaOpenGLContext) << "Beginning frame for" << this
+        << "in" << QThread::currentThread() << "for" << surface;
+
+    Q_ASSERT(surface->surface()->supportsOpenGL());
+
+    if (surface->surface()->surfaceClass() == QSurface::Window) {
+        if (m_needsUpdate.fetchAndStoreRelaxed(false))
+            update();
+    }
+}
+
 /*!
     Sets the drawable object of the NSOpenGLContext, which is the
     frame buffer that is the target of OpenGL drawing operations.