QIcon: prefer downscaling over upscaling pixmap icons

... instead of just choosing the closest scaling factor.
Otherwise we end up with the 1x pixmap for a 1.25 scaling factor instead
of a the better fitting 2x pixmap.

Task-number: QTBUG-90634
Change-Id: Ic554fc8d2715deea43bc22e71414902a263b2fef
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

2 files changed, 21 insertions, 0 deletions

diff --git a/src/gui/image/qicon.cpp b/src/gui/image/qicon.cpp
index 41f9e7541e..35f6cad4e2 100644
--- a/src/gui/image/qicon.cpp
+++ b/src/gui/image/qicon.cpp
@@ -169,6 +169,9 @@ static QPixmapIconEngineEntry *bestSizeScaleMatch(const QSize &size, qreal scale
         qreal ascore = pa->scale - scale;
         qreal bscore = pb->scale - scale;
 
+        // always prefer positive scores to prevent upscaling
+        if ((ascore < 0) != (bscore < 0))
+            return bscore < 0 ? pa : pb;
         // Take the one closest to 0
         return (qAbs(ascore) < qAbs(bscore)) ? pa : pb;
     }
diff --git a/tests/auto/gui/image/qiconhighdpi/tst_qiconhighdpi.cpp b/tests/auto/gui/image/qiconhighdpi/tst_qiconhighdpi.cpp
index 829c463c6b..ca0af5d983 100644
--- a/tests/auto/gui/image/qiconhighdpi/tst_qiconhighdpi.cpp
+++ b/tests/auto/gui/image/qiconhighdpi/tst_qiconhighdpi.cpp
@@ -17,6 +17,7 @@ private slots:
     void addPixmap_data();
     void addPixmap();
     void ninePatch();
+    void preferUpscale();
 };
 
 tst_QIconHighDpi::tst_QIconHighDpi()
@@ -211,6 +212,23 @@ void tst_QIconHighDpi::ninePatch()
     }
 }
 
+void tst_QIconHighDpi::preferUpscale()
+{
+    QIcon icon;
+
+    // manual pixmap adder for full control of devicePixelRatio
+    auto addPixmapWithDpr = [&icon](const QString &path, qreal dpr) {
+        QImage image(path);
+        image.setDevicePixelRatio(dpr);
+        icon.addPixmap(QPixmap::fromImage(image));
+    };
+
+    addPixmapWithDpr(":/icons/testtheme/22x22/actions/appointment-new.png", 1);
+    addPixmapWithDpr(":/icons/testtheme/22x22@2/actions/appointment-new.png", 2);
+
+    QCOMPARE(icon.pixmap(QSize(22, 22), 1.25f).devicePixelRatio(), 1.25f);
+}
+
 int main(int argc, char *argv[])
 {
     QGuiApplication app(argc, argv);