Do not change depth for the backingstore's underlying image

The depth cannot change. This means that RGB16 cannot be upgraded
to 8565 for example as that would be a 24 bit format whereas the
backingstores and the underlying platform may expect a 16 bit
format.

Task-number: QTBUG-50869
Change-Id: I648b39287d43a80fae8097a33bbf3b8bbdcb8816
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

3 files changed, 8 insertions, 2 deletions

diff --git a/src/gui/image/qimage_p.h b/src/gui/image/qimage_p.h
index f9ad6c0ac0..0c0653e8ab 100644
--- a/src/gui/image/qimage_p.h
+++ b/src/gui/image/qimage_p.h
@@ -189,6 +189,12 @@ inline QImage::Format qt_alphaVersion(QImage::Format format)
     return QImage::Format_ARGB32_Premultiplied;
 }
 
+inline QImage::Format qt_maybeAlphaVersionWithSameDepth(QImage::Format format)
+{
+    const QImage::Format toFormat = qt_alphaVersion(format);
+    return qt_depthForFormat(format) == qt_depthForFormat(toFormat) ? toFormat : format;
+}
+
 inline QImage::Format qt_alphaVersionForPainting(QImage::Format format)
 {
     QImage::Format toFormat = qt_alphaVersion(format);
diff --git a/src/plugins/platforms/windows/qwindowsbackingstore.cpp b/src/plugins/platforms/windows/qwindowsbackingstore.cpp
index a6b1d0af26..af9d2a5969 100644
--- a/src/plugins/platforms/windows/qwindowsbackingstore.cpp
+++ b/src/plugins/platforms/windows/qwindowsbackingstore.cpp
@@ -155,7 +155,7 @@ void QWindowsBackingStore::resize(const QSize &size, const QRegion &region)
         if (QImage::toPixelFormat(format).alphaUsage() == QPixelFormat::UsesAlpha)
             m_alphaNeedsFill = true;
         else // upgrade but here we know app painting does not rely on alpha hence no need to fill
-            format = qt_alphaVersionForPainting(format);
+            format = qt_maybeAlphaVersionWithSameDepth(format);
 
         QWindowsNativeImage *oldwni = m_image.data();
         QWindowsNativeImage *newwni = new QWindowsNativeImage(size.width(), size.height(), format);
diff --git a/src/plugins/platforms/xcb/qxcbbackingstore.cpp b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
index c34bea0242..3b04c59e28 100644
--- a/src/plugins/platforms/xcb/qxcbbackingstore.cpp
+++ b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
@@ -179,7 +179,7 @@ QXcbShmImage::QXcbShmImage(QXcbScreen *screen, const QSize &size, uint depth, QI
 
     m_hasAlpha = QImage::toPixelFormat(format).alphaUsage() == QPixelFormat::UsesAlpha;
     if (!m_hasAlpha)
-        format = qt_alphaVersionForPainting(format);
+        format = qt_maybeAlphaVersionWithSameDepth(format);
 
     m_qimage = QImage( (uchar*) m_xcb_image->data, m_xcb_image->width, m_xcb_image->height, m_xcb_image->stride, format);
     m_graphics_buffer = new QXcbShmGraphicsBuffer(&m_qimage);