High-DPI: Set the minimum scale factor to 1

Avoid painting errors with dpr < 1, also for the PassThrough
mode.

This limits the minimum high-dpi scale factor to 1,
for the code path which determines the scale factor
based on screen DPI.

Pick-to: 6.1
Task-number: QTBUG-89948
Change-Id: I14b3f130f0ae141d5f05c87437f926a9f76d1dec
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp b/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
index 09bf8439c8..512209ab73 100644
--- a/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
+++ b/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
@@ -49,6 +49,7 @@ private slots:
     void initTestCase();
     void qhighdpiscaling_data();
     void qhighdpiscaling();
+    void minimumDpr();
     void noscreens();
     void screenDpiAndDpr_data();
     void screenDpiAndDpr();
@@ -223,6 +224,20 @@ void tst_QHighDpi::screenDpiAndDpr()
     }
 }
 
+void tst_QHighDpi::minimumDpr()
+{
+    QList<qreal> dpiValues { 40, 60, 95 };
+    std::unique_ptr<QGuiApplication> app(createStandardOffscreenApp(dpiValues));
+    for (QScreen *screen : app->screens()) {
+        // Qt does not currently support DPR values < 1. Make sure
+        // the minimum DPR value is 1, also when the screen reports
+        // a low DPI.
+        QCOMPARE(screen->devicePixelRatio(), 1);
+        QWindow window(screen);
+        QCOMPARE(window.devicePixelRatio(), 1);
+    }
+}
+
 void tst_QHighDpi::noscreens()
 {
     // Create application object with a no-screens configuration (should not crash)