Fix disfunctional QQuickRenderControl with multiple screens

Having a retina and non-retina screen connected resulted in getting
no output from QQuickRenderControl and QQuickWidget on the
non-retina screen.

This is caused by the fact that Quick is blindly calling
QWindow::devicePixelRatio(). This approach is wrong when using
QQuickRenderControl since the QQuickWindow does not have an actual
native window and so devicePixelRatio() merely returns some default value
which will definitely be wrong for one of the screens.

The patch fixes the problem by introducing
QQuickWindow::effectiveDevicePixelRatio(), which, via
QQuickRenderControl::renderWindowFor, supports the redirected case too.

Task-number: QTBUG-42114
Change-Id: I057e01f0a00758dde438fc9b10af3a435b06bb0b
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

4 files changed, 14 insertions, 5 deletions

diff --git a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
index 802c92be9f..b30a504da9 100644
--- a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
+++ b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
@@ -41,6 +41,7 @@
 #include <private/qfontengine_p.h>
 #include <private/qopenglextensions_p.h>
 
+#include <QtQuick/qquickwindow.h>
 #include <QtQuick/private/qsgtexture_p.h>
 
 #include <private/qrawfont_p.h>
@@ -350,8 +351,16 @@ void QSGTextMaskMaterial::init(QFontEngine::GlyphFormat glyphFormat)
                         : QFontEngine::Format_A32;
         }
 
-        qreal devicePixelRatio = ctx->surface()->surfaceClass() == QSurface::Window ?
-            static_cast<QWindow *>(ctx->surface())->devicePixelRatio() : ctx->screen()->devicePixelRatio();
+        qreal devicePixelRatio;
+        if (ctx->surface()->surfaceClass() == QSurface::Window) {
+            QWindow *w = static_cast<QWindow *>(ctx->surface());
+            if (QQuickWindow *qw = qobject_cast<QQuickWindow *>(w))
+                devicePixelRatio = qw->effectiveDevicePixelRatio();
+            else
+                devicePixelRatio = w->devicePixelRatio();
+        } else {
+            devicePixelRatio = ctx->screen()->devicePixelRatio();
+        }
 
         QTransform glyphCacheTransform = QTransform::fromScale(devicePixelRatio, devicePixelRatio);
         if (!fontEngine->supportsTransformation(glyphCacheTransform))
diff --git a/src/quick/scenegraph/qsgrenderloop.cpp b/src/quick/scenegraph/qsgrenderloop.cpp
index f2586b1e40..c66915d19d 100644
--- a/src/quick/scenegraph/qsgrenderloop.cpp
+++ b/src/quick/scenegraph/qsgrenderloop.cpp
@@ -372,7 +372,7 @@ void QSGGuiThreadRenderLoop::renderWindow(QQuickWindow *window)
         renderTime = renderTimer.nsecsElapsed();
 
     if (data.grabOnly) {
-        grabContent = qt_gl_read_framebuffer(window->size() * window->devicePixelRatio(), false, false);
+        grabContent = qt_gl_read_framebuffer(window->size() * window->effectiveDevicePixelRatio(), false, false);
         data.grabOnly = false;
     }
 
diff --git a/src/quick/scenegraph/qsgthreadedrenderloop.cpp b/src/quick/scenegraph/qsgthreadedrenderloop.cpp
index 7ac5f0a921..155b52b31a 100644
--- a/src/quick/scenegraph/qsgthreadedrenderloop.cpp
+++ b/src/quick/scenegraph/qsgthreadedrenderloop.cpp
@@ -417,7 +417,7 @@ bool QSGRenderThread::event(QEvent *e)
             QQuickWindowPrivate::get(window)->renderSceneGraph(windowSize);
 
             qCDebug(QSG_LOG_RENDERLOOP) << QSG_RT_PAD << "- grabbing result";
-            *ce->image = qt_gl_read_framebuffer(windowSize * window->devicePixelRatio(), false, false);
+            *ce->image = qt_gl_read_framebuffer(windowSize * window->effectiveDevicePixelRatio(), false, false);
         }
         qCDebug(QSG_LOG_RENDERLOOP) << QSG_RT_PAD << "- waking gui to handle result";
         waitCondition.wakeOne();
diff --git a/src/quick/scenegraph/qsgwindowsrenderloop.cpp b/src/quick/scenegraph/qsgwindowsrenderloop.cpp
index 10eba74607..070d6b82fd 100644
--- a/src/quick/scenegraph/qsgwindowsrenderloop.cpp
+++ b/src/quick/scenegraph/qsgwindowsrenderloop.cpp
@@ -286,7 +286,7 @@ QImage QSGWindowsRenderLoop::grab(QQuickWindow *window)
     d->syncSceneGraph();
     d->renderSceneGraph(window->size());
 
-    QImage image = qt_gl_read_framebuffer(window->size() * window->devicePixelRatio(), false, false);
+    QImage image = qt_gl_read_framebuffer(window->size() * window->effectiveDevicePixelRatio(), false, false);
     return image;
 }